diff --git a/src/Adapter/ApplepayAdapter.php b/src/Adapter/ApplepayAdapter.php
index 824650a5e..b23a46366 100644
--- a/src/Adapter/ApplepayAdapter.php
+++ b/src/Adapter/ApplepayAdapter.php
@@ -67,7 +67,7 @@ public function validMerchantValidationDomain(string $merchantValidationURL): bo
      *                             This is necessary if the ssl cert file doesn't contain key already.
      * @param string|null $caCert  Path to CA certificate.
      */
-    public function init(string $sslCert, string $sslKey = null, string $caCert = null): void
+    public function init(string $sslCert, ?string $sslKey = null, ?string $caCert = null): void
     {
         $timeout = EnvironmentService::getTimeout();
         $curlVerbose = EnvironmentService::isCurlVerbose();
diff --git a/src/Adapter/CurlAdapter.php b/src/Adapter/CurlAdapter.php
index e3ddd2b98..5535eb40e 100755
--- a/src/Adapter/CurlAdapter.php
+++ b/src/Adapter/CurlAdapter.php
@@ -10,11 +10,10 @@
 
 namespace UnzerSDK\Adapter;
 
-use UnzerSDK\Unzer;
-use UnzerSDK\Services\EnvironmentService;
-use UnzerSDK\Exceptions\UnzerApiException;
 use RuntimeException;
-
+use UnzerSDK\Exceptions\UnzerApiException;
+use UnzerSDK\Services\EnvironmentService;
+use UnzerSDK\Unzer;
 use function extension_loaded;
 use function in_array;
 
@@ -37,7 +36,7 @@ public function __construct()
     /**
      * {@inheritDoc}
      */
-    public function init(string $url, string $payload = null, string $httpMethod = HttpAdapterInterface::REQUEST_GET): void
+    public function init(string $url, ?string $payload = null, string $httpMethod = HttpAdapterInterface::REQUEST_GET): void
     {
         $timeout = EnvironmentService::getTimeout();
         $curlVerbose = EnvironmentService::isCurlVerbose();
diff --git a/src/Adapter/HttpAdapterInterface.php b/src/Adapter/HttpAdapterInterface.php
index 4e238b87d..2e2bd0a6a 100755
--- a/src/Adapter/HttpAdapterInterface.php
+++ b/src/Adapter/HttpAdapterInterface.php
@@ -25,7 +25,7 @@ interface HttpAdapterInterface
      * @param string|null $payload    Json encoded payload string.
      * @param string      $httpMethod The Http method to perform.
      */
-    public function init(string $url, string $payload = null, string $httpMethod = HttpAdapterInterface::REQUEST_GET): void;
+    public function init(string $url, ?string $payload = null, string $httpMethod = HttpAdapterInterface::REQUEST_GET): void;
 
     /**
      * Executes the request and returns the response.
diff --git a/src/Exceptions/UnzerApiException.php b/src/Exceptions/UnzerApiException.php
index 39d209288..f9e1c44dd 100644
--- a/src/Exceptions/UnzerApiException.php
+++ b/src/Exceptions/UnzerApiException.php
@@ -26,10 +26,10 @@ class UnzerApiException extends Exception
      *
      * @param string      $merchantMessage
      * @param string      $clientMessage
-     * @param string      $code
+     * @param string|null $code
      * @param string|null $errorId
      */
-    public function __construct($merchantMessage = '', $clientMessage = '', $code = null, string $errorId = null)
+    public function __construct($merchantMessage = '', $clientMessage = '', $code = null, ?string $errorId = null)
     {
         $merchantMessage = empty($merchantMessage) ? static::MESSAGE : $merchantMessage;
         $this->clientMessage = empty($clientMessage) ? static::CLIENT_MESSAGE : $clientMessage;
diff --git a/src/Interfaces/CancelServiceInterface.php b/src/Interfaces/CancelServiceInterface.php
index efa5314e9..5400380ad 100644
--- a/src/Interfaces/CancelServiceInterface.php
+++ b/src/Interfaces/CancelServiceInterface.php
@@ -8,13 +8,13 @@
 
 namespace UnzerSDK\Interfaces;
 
+use RuntimeException;
 use UnzerSDK\Constants\CancelReasonCodes;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Resources\Payment;
 use UnzerSDK\Resources\TransactionTypes\Authorization;
 use UnzerSDK\Resources\TransactionTypes\Cancellation;
 use UnzerSDK\Resources\TransactionTypes\Charge;
-use RuntimeException;
 
 interface CancelServiceInterface
 {
@@ -30,7 +30,7 @@ interface CancelServiceInterface
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function cancelAuthorization(Authorization $authorization, float $amount = null): Cancellation;
+    public function cancelAuthorization(Authorization $authorization, ?float $amount = null): Cancellation;
 
     /**
      * Performs a Cancellation transaction for the Authorization of the given Payment object.
@@ -44,7 +44,7 @@ public function cancelAuthorization(Authorization $authorization, float $amount
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function cancelAuthorizationByPayment($payment, float $amount = null): Cancellation;
+    public function cancelAuthorizationByPayment($payment, ?float $amount = null): Cancellation;
 
     /**
      * Performs a Cancellation transaction for the given Charge and returns the resulting Cancellation object.
@@ -67,11 +67,11 @@ public function cancelAuthorizationByPayment($payment, float $amount = null): Ca
     public function cancelChargeById(
         $payment,
         string $chargeId,
-        float $amount = null,
-        string $reasonCode = null,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation;
 
     /**
@@ -93,11 +93,11 @@ public function cancelChargeById(
      */
     public function cancelCharge(
         Charge $charge,
-        float $amount = null,
-        string $reasonCode = null,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation;
 
     /**
@@ -119,11 +119,11 @@ public function cancelCharge(
      */
     public function cancelPayment(
         $payment,
-        float $amount = null,
+        ?float $amount = null,
         ?string $reasonCode = CancelReasonCodes::REASON_CODE_CANCEL,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): array;
 
     /**
@@ -167,5 +167,5 @@ public function cancelChargedPayment($payment, ?Cancellation $cancellation = nul
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is a error while using the SDK.
      */
-    public function cancelPaymentAuthorization($payment, float $amount = null): ?Cancellation;
+    public function cancelPaymentAuthorization($payment, ?float $amount = null): ?Cancellation;
 }
diff --git a/src/Interfaces/PaymentServiceInterface.php b/src/Interfaces/PaymentServiceInterface.php
index 477e03ead..d4613979e 100644
--- a/src/Interfaces/PaymentServiceInterface.php
+++ b/src/Interfaces/PaymentServiceInterface.php
@@ -9,6 +9,7 @@
 namespace UnzerSDK\Interfaces;
 
 use DateTime;
+use RuntimeException;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Resources\AbstractUnzerResource;
 use UnzerSDK\Resources\Basket;
@@ -24,7 +25,6 @@
 use UnzerSDK\Resources\TransactionTypes\Charge;
 use UnzerSDK\Resources\TransactionTypes\Payout;
 use UnzerSDK\Resources\TransactionTypes\Shipment;
-use RuntimeException;
 
 interface PaymentServiceInterface
 {
@@ -48,8 +48,8 @@ public function performAuthorization(
         Authorization $authorization,
         $paymentType,
         $customer = null,
-        Metadata      $metadata = null,
-        Basket        $basket = null
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null
     ): Authorization;
 
     /**
@@ -130,8 +130,8 @@ public function performCharge(
         Charge   $charge,
         $paymentType,
         $customer = null,
-        Metadata $metadata = null,
-        Basket   $basket = null
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null
     ): Charge;
 
     /**
@@ -229,9 +229,9 @@ public function performChargeOnPayment(
      */
     public function chargeAuthorization(
         $payment,
-        float $amount = null,
-        string $orderId = null,
-        string $invoiceId = null
+        ?float $amount = null,
+        ?string $orderId = null,
+        ?string $invoiceId = null
     ): Charge;
 
     /**
@@ -252,9 +252,9 @@ public function chargeAuthorization(
      */
     public function chargePayment(
         $payment,
-        float $amount = null,
-        string $orderId = null,
-        string $invoiceId = null
+        ?float $amount = null,
+        ?string $orderId = null,
+        ?string $invoiceId = null
     ): Charge;
 
     /**
@@ -284,11 +284,11 @@ public function payout(
         $paymentType,
         string   $returnUrl,
         $customer = null,
-        string   $orderId = null,
-        Metadata $metadata = null,
-        Basket   $basket = null,
-        string   $invoiceId = null,
-        string   $referenceText = null
+        ?string   $orderId = null,
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null,
+        ?string   $invoiceId = null,
+        ?string   $referenceText = null
     ): Payout;
 
     /**
@@ -303,7 +303,7 @@ public function payout(
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function ship($payment, string $invoiceId = null, string $orderId = null): Shipment;
+    public function ship($payment, ?string $invoiceId = null, ?string $orderId = null): Shipment;
 
     /**
      * Initializes a PayPage for charge transaction and returns the PayPage resource.
@@ -328,9 +328,9 @@ public function ship($payment, string $invoiceId = null, string $orderId = null)
      */
     public function initPayPageCharge(
         Paypage  $paypage,
-        Customer $customer = null,
-        Basket   $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage;
 
     /**
@@ -356,9 +356,9 @@ public function initPayPageCharge(
      */
     public function initPayPageAuthorize(
         Paypage  $paypage,
-        Customer $customer = null,
-        Basket   $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage;
 
     /**
@@ -378,7 +378,7 @@ public function fetchInstallmentPlans(
         float    $amount,
         string   $currency,
         float    $effectiveInterest,
-        DateTime $orderDate = null
+        ?DateTime $orderDate = null
     ): InstalmentPlans;
 
     /**
diff --git a/src/Interfaces/ResourceServiceInterface.php b/src/Interfaces/ResourceServiceInterface.php
index 531470dd8..6f26b0ad1 100644
--- a/src/Interfaces/ResourceServiceInterface.php
+++ b/src/Interfaces/ResourceServiceInterface.php
@@ -57,7 +57,7 @@ public function fetchPayout($payment): Payout;
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function activateRecurringPayment($paymentType, string $returnUrl, string $recurrenceType = null): Recurring;
+    public function activateRecurringPayment($paymentType, string $returnUrl, ?string $recurrenceType = null): Recurring;
 
     /**
      * Fetch and return payment by given payment id or payment object.
diff --git a/src/Interfaces/WebhookServiceInterface.php b/src/Interfaces/WebhookServiceInterface.php
index 48c89e2eb..8ae6b5a03 100644
--- a/src/Interfaces/WebhookServiceInterface.php
+++ b/src/Interfaces/WebhookServiceInterface.php
@@ -8,10 +8,10 @@
 
 namespace UnzerSDK\Interfaces;
 
+use RuntimeException;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Resources\AbstractUnzerResource;
 use UnzerSDK\Resources\Webhook;
-use RuntimeException;
 
 interface WebhookServiceInterface
 {
@@ -106,5 +106,5 @@ public function registerMultipleWebhooks(string $url, array $events): array;
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function fetchResourceFromEvent(string $eventJson = null): AbstractUnzerResource;
+    public function fetchResourceFromEvent(?string $eventJson = null): AbstractUnzerResource;
 }
diff --git a/src/Resources/EmbeddedResources/GooglePay/SignedMessage.php b/src/Resources/EmbeddedResources/GooglePay/SignedMessage.php
index 647474da1..31dfa913c 100644
--- a/src/Resources/EmbeddedResources/GooglePay/SignedMessage.php
+++ b/src/Resources/EmbeddedResources/GooglePay/SignedMessage.php
@@ -15,12 +15,7 @@ class SignedMessage extends AbstractUnzerResource
     /** @var string */
     protected $encryptedMessage;
 
-    /**
-     * @param string $tag
-     * @param string $ephemeralPublicKey
-     * @param string $encryptedMessage
-     */
-    public function __construct(string $tag = null, string $ephemeralPublicKey = null, string $encryptedMessage = null)
+    public function __construct(?string $tag = null, ?string $ephemeralPublicKey = null, ?string $encryptedMessage = null)
     {
         $this->tag = $tag;
         $this->ephemeralPublicKey = $ephemeralPublicKey;
diff --git a/src/Resources/InstalmentPlan.php b/src/Resources/InstalmentPlan.php
index bcdcf8591..3c9769548 100644
--- a/src/Resources/InstalmentPlan.php
+++ b/src/Resources/InstalmentPlan.php
@@ -3,10 +3,10 @@
 namespace UnzerSDK\Resources;
 
 use DateTime;
+use stdClass;
 use UnzerSDK\Adapter\HttpAdapterInterface;
 use UnzerSDK\Resources\PaymentTypes\BasePaymentType;
 use UnzerSDK\Traits\CanAuthorizeWithCustomer;
-use stdClass;
 
 /**
  * Resource representing the installment plan for Installment Secured.
@@ -77,17 +77,17 @@ class InstalmentPlan extends BasePaymentType
      * @param float|null  $lastRate
      */
     public function __construct(
-        int    $numberOfRates = null,
-        string $dayOfPurchase = null,
-        float  $totalPurchaseAmount = null,
-        float  $totalInterestAmount = null,
-        float  $totalAmount = null,
-        float  $effectiveInterestRate = null,
-        float  $nominalInterestRate = null,
-        float  $feeFirstRate = null,
-        float  $feePerRate = null,
-        float  $monthlyRate = null,
-        float  $lastRate = null
+        ?int   $numberOfRates = null,
+        ?string $dayOfPurchase = null,
+        ?float $totalPurchaseAmount = null,
+        ?float $totalInterestAmount = null,
+        ?float $totalAmount = null,
+        ?float $effectiveInterestRate = null,
+        ?float $nominalInterestRate = null,
+        ?float $feeFirstRate = null,
+        ?float $feePerRate = null,
+        ?float $monthlyRate = null,
+        ?float $lastRate = null
     ) {
         $this->numberOfRates         = $numberOfRates;
         $this->dayOfPurchase         = $dayOfPurchase;
diff --git a/src/Resources/Payment.php b/src/Resources/Payment.php
index 20f956d32..4a75656b4 100755
--- a/src/Resources/Payment.php
+++ b/src/Resources/Payment.php
@@ -829,11 +829,11 @@ public function getExternalId(): ?string
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
     public function cancelAmount(
-        float   $amount = null,
+        ?float  $amount = null,
         ?string $reasonCode = CancelReasonCodes::REASON_CODE_CANCEL,
-        string  $paymentReference = null,
-        float   $amountNet = null,
-        float   $amountVat = null
+        ?string $paymentReference = null,
+        ?float  $amountNet = null,
+        ?float  $amountVat = null
     ): array {
         return $this->getUnzerObject()->cancelPayment($this, $amount, $reasonCode, $paymentReference, $amountNet, $amountVat);
     }
@@ -850,7 +850,7 @@ public function cancelAmount(
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function cancelAuthorizationAmount(float $amount = null): ?Cancellation
+    public function cancelAuthorizationAmount(?float $amount = null): ?Cancellation
     {
         return $this->getUnzerObject()->cancelPaymentAuthorization($this, $amount);
     }
@@ -865,7 +865,7 @@ public function cancelAuthorizationAmount(float $amount = null): ?Cancellation
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function charge(float $amount = null): Charge
+    public function charge(?float $amount = null): Charge
     {
         return $this->getUnzerObject()->chargePayment($this, $amount);
     }
@@ -881,7 +881,7 @@ public function charge(float $amount = null): Charge
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function ship(string $invoiceId = null, string $orderId = null)
+    public function ship(?string $invoiceId = null, ?string $orderId = null)
     {
         return $this->getUnzerObject()->ship($this, $invoiceId, $orderId);
     }
diff --git a/src/Resources/PaymentTypes/Card.php b/src/Resources/PaymentTypes/Card.php
index 3a6888838..3ada8c562 100755
--- a/src/Resources/PaymentTypes/Card.php
+++ b/src/Resources/PaymentTypes/Card.php
@@ -2,6 +2,8 @@
 
 namespace UnzerSDK\Resources\PaymentTypes;
 
+use RuntimeException;
+use stdClass;
 use UnzerSDK\Adapter\HttpAdapterInterface;
 use UnzerSDK\Resources\EmbeddedResources\CardDetails;
 use UnzerSDK\Traits\CanAuthorize;
@@ -9,8 +11,6 @@
 use UnzerSDK\Traits\CanPayout;
 use UnzerSDK\Traits\CanRecur;
 use UnzerSDK\Validators\ExpiryDateValidator;
-use RuntimeException;
-use stdClass;
 
 /**
  * This represents the card payment type which supports credit card as well as debit card payments.
@@ -56,7 +56,7 @@ class Card extends BasePaymentType
      * @param string|null $expiryDate
      * @param string|null $email
      */
-    public function __construct(?string $number, ?string $expiryDate, string $email = null)
+    public function __construct(?string $number, ?string $expiryDate, ?string $email = null)
     {
         $this->setNumber($number);
         $this->setExpiryDate($expiryDate);
diff --git a/src/Resources/PaymentTypes/InstallmentSecured.php b/src/Resources/PaymentTypes/InstallmentSecured.php
index 1410d7130..15a1dd04a 100644
--- a/src/Resources/PaymentTypes/InstallmentSecured.php
+++ b/src/Resources/PaymentTypes/InstallmentSecured.php
@@ -34,7 +34,7 @@ class InstallmentSecured extends InstalmentPlan
      * @param null|DateTime|string $invoiceDate
      * @param null|DateTime|string $invoiceDueDate
      */
-    public function __construct(InstalmentPlan $selectedPlan = null, $iban = null, $accountHolder = null, $orderDate = null, $bic = null, $invoiceDate = null, $invoiceDueDate = null)
+    public function __construct(?InstalmentPlan $selectedPlan = null, $iban = null, $accountHolder = null, $orderDate = null, $bic = null, $invoiceDate = null, $invoiceDueDate = null)
     {
         parent::__construct();
 
diff --git a/src/Resources/PaymentTypes/OpenbankingPis.php b/src/Resources/PaymentTypes/OpenbankingPis.php
index f8e07de48..c7fa5af34 100644
--- a/src/Resources/PaymentTypes/OpenbankingPis.php
+++ b/src/Resources/PaymentTypes/OpenbankingPis.php
@@ -9,7 +9,7 @@ class OpenbankingPis extends BasePaymentType
     /** @var string|null $ibanCountry */
     protected $ibanCountry;
 
-    public function __construct(string $ibanCountry = null)
+    public function __construct(?string $ibanCountry = null)
     {
         $this->ibanCountry = $ibanCountry;
     }
diff --git a/src/Resources/PaymentTypes/PaylaterDirectDebit.php b/src/Resources/PaymentTypes/PaylaterDirectDebit.php
index fb7c3a179..8c89b5988 100644
--- a/src/Resources/PaymentTypes/PaylaterDirectDebit.php
+++ b/src/Resources/PaymentTypes/PaylaterDirectDebit.php
@@ -12,7 +12,7 @@ class PaylaterDirectDebit extends BasePaymentType
     /** @var string $holder */
     protected $holder;
 
-    public function __construct(string $iban = null, string $holder = null)
+    public function __construct(?string $iban = null, ?string $holder = null)
     {
         $this->iban = $iban;
         $this->holder = $holder;
diff --git a/src/Resources/TransactionTypes/Authorization.php b/src/Resources/TransactionTypes/Authorization.php
index b37ac71d2..6936b8b84 100755
--- a/src/Resources/TransactionTypes/Authorization.php
+++ b/src/Resources/TransactionTypes/Authorization.php
@@ -2,6 +2,7 @@
 
 namespace UnzerSDK\Resources\TransactionTypes;
 
+use RuntimeException;
 use UnzerSDK\Adapter\HttpAdapterInterface;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Resources\Payment;
@@ -9,7 +10,6 @@
 use UnzerSDK\Traits\HasCancellations;
 use UnzerSDK\Traits\HasDescriptor;
 use UnzerSDK\Traits\HasRecurrenceType;
-use RuntimeException;
 
 /**
  * This represents the authorization transaction.
@@ -55,7 +55,7 @@ class Authorization extends AbstractTransactionType
      * @param string|null $currency
      * @param string|null $returnUrl
      */
-    public function __construct(float $amount = null, string $currency = null, string $returnUrl = null)
+    public function __construct(?float $amount = null, ?string $currency = null, ?string $returnUrl = null)
     {
         $this->setAmount($amount);
         $this->setCurrency($currency);
@@ -250,7 +250,7 @@ protected function getResourcePath(string $httpMethod = HttpAdapterInterface::RE
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function cancel(float $amount = null): Cancellation
+    public function cancel(?float $amount = null): Cancellation
     {
         return $this->getUnzerObject()->cancelAuthorization($this, $amount);
     }
@@ -265,7 +265,7 @@ public function cancel(float $amount = null): Cancellation
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function charge(float $amount = null): Charge
+    public function charge(?float $amount = null): Charge
     {
         $payment = $this->getPayment();
         if (!$payment instanceof Payment) {
diff --git a/src/Resources/TransactionTypes/Cancellation.php b/src/Resources/TransactionTypes/Cancellation.php
index 26d92e851..ad41d1c22 100755
--- a/src/Resources/TransactionTypes/Cancellation.php
+++ b/src/Resources/TransactionTypes/Cancellation.php
@@ -6,7 +6,6 @@
 use UnzerSDK\Constants\CancelReasonCodes;
 use UnzerSDK\Resources\Payment;
 use UnzerSDK\Resources\PaymentTypes\InstallmentSecured;
-
 use function in_array;
 
 /**
@@ -49,7 +48,7 @@ class Cancellation extends AbstractTransactionType
      *
      * @param float|null $amount The amount to be cancelled, is transferred as grossAmount in case of Installment Secured.
      */
-    public function __construct(float $amount = null)
+    public function __construct(?float $amount = null)
     {
         $this->setAmount($amount);
     }
diff --git a/src/Resources/TransactionTypes/Charge.php b/src/Resources/TransactionTypes/Charge.php
index d7f16858a..278cc9c3b 100755
--- a/src/Resources/TransactionTypes/Charge.php
+++ b/src/Resources/TransactionTypes/Charge.php
@@ -2,6 +2,7 @@
 
 namespace UnzerSDK\Resources\TransactionTypes;
 
+use RuntimeException;
 use UnzerSDK\Adapter\HttpAdapterInterface;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Traits\HasAccountInformation;
@@ -9,7 +10,6 @@
 use UnzerSDK\Traits\HasChargebacks;
 use UnzerSDK\Traits\HasDescriptor;
 use UnzerSDK\Traits\HasRecurrenceType;
-use RuntimeException;
 
 /**
  * This represents the charge transaction.
@@ -47,7 +47,7 @@ class Charge extends AbstractTransactionType
      * @param string|null $currency
      * @param string|null $returnUrl
      */
-    public function __construct(float $amount = null, string $currency = null, string $returnUrl = null)
+    public function __construct(?float $amount = null, ?string $currency = null, ?string $returnUrl = null)
     {
         $this->setAmount($amount);
         $this->setCurrency($currency);
@@ -199,11 +199,11 @@ protected function getResourcePath(string $httpMethod = HttpAdapterInterface::RE
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
     public function cancel(
-        float  $amount = null,
-        string $reasonCode = null,
-        string $paymentReference = null,
-        float  $amountNet = null,
-        float  $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $paymentReference = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation {
         return $this->getUnzerObject()->cancelCharge(
             $this,
diff --git a/src/Resources/TransactionTypes/Chargeback.php b/src/Resources/TransactionTypes/Chargeback.php
index 7c0c5925a..2ef22d038 100644
--- a/src/Resources/TransactionTypes/Chargeback.php
+++ b/src/Resources/TransactionTypes/Chargeback.php
@@ -24,7 +24,7 @@ class Chargeback extends AbstractTransactionType
     /**
      * @param float|null $amount The amount to be cancelled, is transferred as grossAmount in case of Installment Secured.
      */
-    public function __construct(float $amount = null)
+    public function __construct(?float $amount = null)
     {
         $this->setAmount($amount);
     }
diff --git a/src/Resources/TransactionTypes/Payout.php b/src/Resources/TransactionTypes/Payout.php
index 39f6a93de..15f0767ca 100644
--- a/src/Resources/TransactionTypes/Payout.php
+++ b/src/Resources/TransactionTypes/Payout.php
@@ -24,14 +24,7 @@ class Payout extends AbstractTransactionType
     /** @var string $paymentReference */
     protected $paymentReference;
 
-    /**
-     * Payout constructor.
-     *
-     * @param float|null  $amount
-     * @param string|null $currency
-     * @param null        $returnUrl
-     */
-    public function __construct(float $amount = null, string $currency = null, $returnUrl = null)
+    public function __construct(?float $amount = null, ?string $currency = null, $returnUrl = null)
     {
         $this->setAmount($amount);
         $this->setCurrency($currency);
diff --git a/src/Services/CancelService.php b/src/Services/CancelService.php
index 354cb9011..37106177c 100644
--- a/src/Services/CancelService.php
+++ b/src/Services/CancelService.php
@@ -12,7 +12,6 @@
 use UnzerSDK\Resources\TransactionTypes\Cancellation;
 use UnzerSDK\Resources\TransactionTypes\Charge;
 use UnzerSDK\Unzer;
-
 use function in_array;
 use function is_string;
 
@@ -67,7 +66,7 @@ public function getResourceService(): ResourceService
     /**
      * {@inheritDoc}
      */
-    public function cancelAuthorization(Authorization $authorization, float $amount = null): Cancellation
+    public function cancelAuthorization(Authorization $authorization, ?float $amount = null): Cancellation
     {
         $cancellation = new Cancellation($amount);
         $cancellation->setPayment($authorization->getPayment())->setParentResource($authorization);
@@ -80,7 +79,7 @@ public function cancelAuthorization(Authorization $authorization, float $amount
     /**
      * {@inheritDoc}
      */
-    public function cancelAuthorizationByPayment($payment, float $amount = null): Cancellation
+    public function cancelAuthorizationByPayment($payment, ?float $amount = null): Cancellation
     {
         $authorization = $this->getResourceService()->fetchAuthorization($payment);
         return $this->cancelAuthorization($authorization, $amount);
@@ -92,11 +91,11 @@ public function cancelAuthorizationByPayment($payment, float $amount = null): Ca
     public function cancelChargeById(
         $payment,
         string $chargeId,
-        float $amount = null,
-        string $reasonCode = null,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation {
         $charge = $this->getResourceService()->fetchChargeById($payment, $chargeId);
         return $this->cancelCharge($charge, $amount, $reasonCode, $referenceText, $amountNet, $amountVat);
@@ -107,11 +106,11 @@ public function cancelChargeById(
      */
     public function cancelCharge(
         Charge $charge,
-        float $amount = null,
-        string $reasonCode = null,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation {
         $cancellation = new Cancellation($amount);
         $cancellation
@@ -131,11 +130,11 @@ public function cancelCharge(
      */
     public function cancelPayment(
         $payment,
-        float $amount = null,
+        ?float $amount = null,
         ?string $reasonCode = CancelReasonCodes::REASON_CODE_CANCEL,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): array {
         $paymentObject = $payment;
         if (is_string($payment)) {
@@ -186,7 +185,7 @@ public function cancelPayment(
     /**
      * {@inheritDoc}
      */
-    public function cancelPaymentAuthorization($payment, float $amount = null): ?Cancellation
+    public function cancelPaymentAuthorization($payment, ?float $amount = null): ?Cancellation
     {
         $cancellation   = null;
         $completeCancel = $amount === null;
diff --git a/src/Services/HttpService.php b/src/Services/HttpService.php
index 4e4fb0604..cce644cb5 100755
--- a/src/Services/HttpService.php
+++ b/src/Services/HttpService.php
@@ -18,7 +18,6 @@
  * This service provides for functionalities concerning http transactions.
  *
  * @link  https://docs.unzer.com/
- *
  */
 class HttpService
 {
@@ -82,26 +81,26 @@ public function setEnvironmentService(EnvironmentService $environmentService): H
     }
 
     /**
-     * @deprecated use sendRequest() instead.
-     *
      * send post request to payment server
      *
-     * @param                        $uri        string|null uri of the target system
-     * @param ?AbstractUnzerResource $resource
-     * @param string                 $httpMethod
-     * @param string                 $apiVersion
+     * @param string|null $uri uri of the target system
+     * @param AbstractUnzerResource|null $resource
+     * @param string $httpMethod
+     * @param string|null $apiVersion
      *
      * @return string
      *
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
+     * @deprecated use sendRequest() instead.
      */
     public function send(
         ?string                $uri = null,
         ?AbstractUnzerResource $resource = null,
         string                 $httpMethod = HttpAdapterInterface::REQUEST_GET,
-        string $apiVersion = null
-    ): string {
+        ?string $apiVersion = null
+    ): string
+    {
         if (!$resource instanceof AbstractUnzerResource) {
             throw new RuntimeException('Transfer object is empty!');
         }
@@ -115,15 +114,11 @@ public function send(
     /**
      * send post request to payment server
      *
-     * @param ApiRequest $request
-     * @return string
-     *
      * @throws UnzerApiException An UnzerApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function sendRequest(
-        ApiRequest $request
-    ): string {
+    public function sendRequest(ApiRequest $request): string
+    {
         $unzerObj = $request->getResource()->getUnzerObject();
 
         $apiConfig = $request->getResource()->getApiConfig();
@@ -137,8 +132,8 @@ public function sendRequest(
         $headers = $this->composeHttpHeaders($unzerObj, $apiConfig::getAuthorizationMethod());
         $httpMethod = $request->getHttpMethod();
         $this->initRequest($requestUrl, $payload, $httpMethod, $headers);
-        $httpAdapter  = $this->getAdapter();
-        $response     = $httpAdapter->execute();
+        $httpAdapter = $this->getAdapter();
+        $response = $httpAdapter->execute();
         $responseCode = $httpAdapter->getResponseCode();
         $httpAdapter->close();
 
@@ -158,7 +153,7 @@ public function sendRequest(
      * @param string $uri
      * @param string $payload
      * @param string $httpMethod
-     * @param array  $httpHeaders
+     * @param array $httpHeaders
      *
      * @throws RuntimeException
      */
@@ -174,7 +169,7 @@ private function initRequest(string $uri, string $payload, string $httpMethod, a
      * Handles error responses by throwing an UnzerApiException with the returned messages and error code.
      * Returns doing nothing if no error occurred.
      *
-     * @param string      $responseCode
+     * @param string $responseCode
      * @param string|null $response
      *
      * @throws UnzerApiException
@@ -187,15 +182,15 @@ private function handleErrors(string $responseCode, ?string $response): void
 
         $responseObject = json_decode($response, false);
         if (!is_numeric($responseCode) || (int)$responseCode >= 400 || isset($responseObject->errors)) {
-            $code            = null;
-            $errorId         = null;
+            $code = null;
+            $errorId = null;
             $customerMessage = $code;
             $merchantMessage = $customerMessage;
             if (isset($responseObject->errors[0])) {
-                $errors          = $responseObject->errors[0];
+                $errors = $responseObject->errors[0];
                 $merchantMessage = $errors->merchantMessage ?? '';
                 $customerMessage = $errors->customerMessage ?? '';
-                $code            = $errors->code ?? '';
+                $code = $errors->code ?? '';
             }
             if (isset($responseObject->id)) {
                 $errorId = $responseObject->id;
@@ -209,23 +204,24 @@ private function handleErrors(string $responseCode, ?string $response): void
     }
 
     /**
-     * @param Unzer       $unzerObj
-     * @param string      $payload
-     * @param mixed       $headers
-     * @param string      $responseCode
-     * @param string      $httpMethod
-     * @param string      $url
+     * @param Unzer $unzerObj
+     * @param string $payload
+     * @param mixed $headers
+     * @param string $responseCode
+     * @param string $httpMethod
+     * @param string $url
      * @param string|null $response
      */
     public function debugLog(
         Unzer  $unzerObj,
         string $payload,
-        $headers,
-        string    $responseCode,
+               $headers,
+        string $responseCode,
         string $httpMethod,
         string $url,
         ?string $response
-    ): void {
+    ): void
+    {
         // mask auth string
         $authHeader = explode(' ', $headers['Authorization']);
         $authHeader[1] = ValueService::maskValue($authHeader[1]);
@@ -275,16 +271,16 @@ private function buildRequestUrl(ApiRequest $request): string
      */
     public function composeHttpHeaders(Unzer $unzer, string $authorizationMethod = AuthorizationMethods::BASIC): array
     {
-        $locale      = $unzer->getLocale();
-        $clientIp    = $unzer->getClientIp();
-        $key         = $unzer->getKey();
+        $locale = $unzer->getLocale();
+        $clientIp = $unzer->getClientIp();
+        $key = $unzer->getKey();
 
         $httpHeaders = [
             'Authorization' => $this->findAuthentication($unzer, $authorizationMethod),
-            'Content-Type'  => 'application/json',
-            'SDK-VERSION'   => Unzer::SDK_VERSION,
-            'SDK-TYPE'      => Unzer::SDK_TYPE,
-            'PHP-VERSION'   => PHP_VERSION
+            'Content-Type' => 'application/json',
+            'SDK-VERSION' => Unzer::SDK_VERSION,
+            'SDK-TYPE' => Unzer::SDK_TYPE,
+            'PHP-VERSION' => PHP_VERSION
         ];
         if (!empty($locale)) {
             $httpHeaders['Accept-Language'] = $locale;
@@ -347,6 +343,7 @@ public function isApiConfig($className): bool
 
     /**
      * @param Unzer $unzer
+     *
      * @return string
      */
     private function findAuthentication(Unzer $unzer, string $authorizationMethod = AuthorizationMethods::BASIC): string
diff --git a/src/Services/PaymentService.php b/src/Services/PaymentService.php
index c59cd819d..8eefc3597 100755
--- a/src/Services/PaymentService.php
+++ b/src/Services/PaymentService.php
@@ -73,12 +73,15 @@ public function getResourceService(): ResourceService
         return $this->getUnzer()->getResourceService();
     }
 
+    /**
+     * {@inheritDoc}
+     */
     public function performAuthorization(
         Authorization $authorization,
         $paymentType,
         $customer = null,
-        Metadata $metadata = null,
-        Basket $basket = null
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null
     ): Authorization {
         $payment = $this->createPayment($paymentType);
         $paymentType = $payment->getPaymentType();
@@ -144,7 +147,7 @@ public function authorize(
     /**
      * {@inheritDoc}
      */
-    public function performCharge(Charge $charge, $paymentType, $customer = null, Metadata $metadata = null, Basket $basket = null): Charge
+    public function performCharge(Charge $charge, $paymentType, $customer = null, ?Metadata $metadata = null, ?Basket $basket = null): Charge
     {
         $payment     = $this->createPayment($paymentType);
         $paymentType = $payment->getPaymentType();
@@ -214,9 +217,9 @@ public function charge(
      */
     public function chargeAuthorization(
         $payment,
-        float $amount = null,
-        string $orderId = null,
-        string $invoiceId = null
+        ?float $amount = null,
+        ?string $orderId = null,
+        ?string $invoiceId = null
     ): Charge {
         return $this->chargePayment($payment, $amount, $orderId, $invoiceId);
     }
@@ -226,9 +229,9 @@ public function chargeAuthorization(
      */
     public function chargePayment(
         $payment,
-        float $amount = null,
-        string $orderId = null,
-        string $invoiceId = null
+        ?float $amount = null,
+        ?string $orderId = null,
+        ?string $invoiceId = null
     ): Charge {
         $charge = new Charge($amount);
 
@@ -263,11 +266,11 @@ public function payout(
         $paymentType,
         string   $returnUrl,
         $customer = null,
-        string   $orderId = null,
-        Metadata $metadata = null,
-        Basket   $basket = null,
-        string   $invoiceId = null,
-        string $referenceText = null
+        ?string   $orderId = null,
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null,
+        ?string   $invoiceId = null,
+        ?string $referenceText = null
     ): Payout {
         $payment = $this->createPayment($paymentType);
         $payout = (new Payout($amount, $currency, $returnUrl))
@@ -283,7 +286,7 @@ public function payout(
     /**
      * {@inheritDoc}
      */
-    public function ship($payment, string $invoiceId = null, string $orderId = null): Shipment
+    public function ship($payment, ?string $invoiceId = null, ?string $orderId = null): Shipment
     {
         $shipment = new Shipment();
         $shipment->setInvoiceId($invoiceId)->setOrderId($orderId);
@@ -297,9 +300,9 @@ public function ship($payment, string $invoiceId = null, string $orderId = null)
      */
     public function initPayPageCharge(
         Paypage $paypage,
-        Customer $customer = null,
-        Basket $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage {
         return $this->initPayPage($paypage, TransactionTypes::CHARGE, $customer, $basket, $metadata);
     }
@@ -309,9 +312,9 @@ public function initPayPageCharge(
      */
     public function initPayPageAuthorize(
         Paypage $paypage,
-        Customer $customer = null,
-        Basket $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage {
         return $this->initPayPage($paypage, TransactionTypes::AUTHORIZATION, $customer, $basket, $metadata);
     }
@@ -323,7 +326,7 @@ public function fetchInstallmentPlans(
         float    $amount,
         string   $currency,
         float    $effectiveInterest,
-        DateTime $orderDate = null
+        ?DateTime $orderDate = null
     ): InstalmentPlans {
         $ins   = (new InstallmentSecured(null, null, null))->setParentResource($this->unzer);
         $plans = (new InstalmentPlans($amount, $currency, $effectiveInterest, $orderDate))->setParentResource($ins);
@@ -365,9 +368,9 @@ public function fetchPaylaterInstallmentPlans(
     private function initPayPage(
         Paypage  $paypage,
         string   $action,
-        Customer $customer = null,
-        Basket   $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage {
         $paypage->setAction($action)->setParentResource($this->unzer);
         $payment = $this->createPayment($paypage)->setBasket($basket)->setCustomer($customer)->setMetadata($metadata)->setPayPage($paypage);
diff --git a/src/Services/ResourceService.php b/src/Services/ResourceService.php
index 7436fb564..2b2d1c922 100755
--- a/src/Services/ResourceService.php
+++ b/src/Services/ResourceService.php
@@ -110,7 +110,7 @@ public function setUnzer(Unzer $unzer): ResourceServiceInterface
      *
      * @param AbstractUnzerResource $resource
      * @param string $httpMethod
-     * @param string $apiVersion
+     * @param string|null $apiVersion
      *
      * @return stdClass
      *
@@ -120,7 +120,7 @@ public function setUnzer(Unzer $unzer): ResourceServiceInterface
     public function send(
         AbstractUnzerResource $resource,
         string                $httpMethod = HttpAdapterInterface::REQUEST_GET,
-        string $apiVersion = null
+        ?string $apiVersion = null
     ): stdClass
     {
         $apiConfig = $resource->getApiConfig();
@@ -338,7 +338,7 @@ public function deleteResource(AbstractUnzerResource &$resource): ?AbstractUnzer
      * Updates the given local resource object (id must be set)
      *
      * @param AbstractUnzerResource $resource The local resource object to update.
-     * @param string $apiVersion
+     * @param string|null $apiVersion
      *
      * @return AbstractUnzerResource The updated resource object.
      *
@@ -346,7 +346,7 @@ public function deleteResource(AbstractUnzerResource &$resource): ?AbstractUnzer
      * @throws RuntimeException  A RuntimeException is thrown when there is an error while using the SDK.
      * @throws Exception
      */
-    public function fetchResource(AbstractUnzerResource $resource, string $apiVersion = null): AbstractUnzerResource
+    public function fetchResource(AbstractUnzerResource $resource, ?string $apiVersion = null): AbstractUnzerResource
     {
         $method = HttpAdapterInterface::REQUEST_GET;
         $response = $this->send($resource, $method, $apiVersion);
@@ -379,7 +379,7 @@ public function fetchPayout($payment): Payout
     /**
      * {@inheritDoc}
      */
-    public function activateRecurringPayment($paymentType, string $returnUrl, string $recurrenceType = null): Recurring
+    public function activateRecurringPayment($paymentType, string $returnUrl, ?string $recurrenceType = null): Recurring
     {
         $paymentTypeObject = $paymentType;
         if (is_string($paymentType)) {
@@ -566,7 +566,7 @@ public function fetchBasket($basket): Basket
         try {
             $this->fetchResource($basketObj, $basketVersion);
         } catch (UnzerApiException $exception) {
-            if ($exception->getCode() !== ApiResponseCodes::API_ERROR_BASKET_NOT_FOUND || $isV3Basket) {
+            if ($exception->getCode() !== ApiResponseCodes::API_ERROR_BASKET_NOT_FOUND || $basketVersion === ApiVersions::V3) {
                 throw $exception;
             }
             $this->fetchResource($basketObj);
diff --git a/src/Services/WebhookService.php b/src/Services/WebhookService.php
index c8117d185..a273bb567 100755
--- a/src/Services/WebhookService.php
+++ b/src/Services/WebhookService.php
@@ -2,14 +2,13 @@
 
 namespace UnzerSDK\Services;
 
-use UnzerSDK\Unzer;
+use RuntimeException;
 use UnzerSDK\Interfaces\ResourceServiceInterface;
 use UnzerSDK\Interfaces\WebhookServiceInterface;
 use UnzerSDK\Resources\AbstractUnzerResource;
 use UnzerSDK\Resources\Webhook;
 use UnzerSDK\Resources\Webhooks;
-use RuntimeException;
-
+use UnzerSDK\Unzer;
 use function is_string;
 
 /**
@@ -165,7 +164,7 @@ public function registerMultipleWebhooks(string $url, array $events): array
     /**
      * {@inheritDoc}
      */
-    public function fetchResourceFromEvent(string $eventJson = null): AbstractUnzerResource
+    public function fetchResourceFromEvent(?string $eventJson = null): AbstractUnzerResource
     {
         $resourceObject = null;
         $eventData = json_decode($eventJson ?? $this->readInputStream(), false);
diff --git a/src/Traits/CanPayout.php b/src/Traits/CanPayout.php
index 4458e68d2..f03042d29 100644
--- a/src/Traits/CanPayout.php
+++ b/src/Traits/CanPayout.php
@@ -8,13 +8,13 @@
 
 namespace UnzerSDK\Traits;
 
+use RuntimeException;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Interfaces\UnzerParentInterface;
 use UnzerSDK\Resources\Basket;
 use UnzerSDK\Resources\Customer;
 use UnzerSDK\Resources\Metadata;
 use UnzerSDK\Resources\TransactionTypes\Payout;
-use RuntimeException;
 
 trait CanPayout
 {
@@ -44,11 +44,11 @@ public function payout(
         string $currency,
         string $returnUrl,
         $customer = null,
-        string $orderId = null,
+        ?string $orderId = null,
         $metadata = null,
-        Basket $basket = null,
-        string $invoiceId = null,
-        string $paymentReference = null
+        ?Basket $basket = null,
+        ?string $invoiceId = null,
+        ?string $paymentReference = null
     ): Payout {
         if ($this instanceof UnzerParentInterface) {
             return $this->getUnzerObject()->payout(
diff --git a/src/Traits/CanPayoutWithCustomer.php b/src/Traits/CanPayoutWithCustomer.php
index 389a4bf06..76c5673e3 100644
--- a/src/Traits/CanPayoutWithCustomer.php
+++ b/src/Traits/CanPayoutWithCustomer.php
@@ -8,13 +8,13 @@
 
 namespace UnzerSDK\Traits;
 
+use RuntimeException;
 use UnzerSDK\Exceptions\UnzerApiException;
 use UnzerSDK\Interfaces\UnzerParentInterface;
 use UnzerSDK\Resources\Basket;
 use UnzerSDK\Resources\Customer;
 use UnzerSDK\Resources\Metadata;
 use UnzerSDK\Resources\TransactionTypes\Payout;
-use RuntimeException;
 
 trait CanPayoutWithCustomer
 {
@@ -42,9 +42,9 @@ public function payout(
         string $currency,
         string $returnUrl,
         $customer,
-        string $orderId = null,
+        ?string $orderId = null,
         $metadata = null,
-        Basket $basket = null
+        ?Basket $basket = null
     ): Payout {
         if ($this instanceof UnzerParentInterface) {
             return $this->getUnzerObject()->payout(
diff --git a/src/Unzer.php b/src/Unzer.php
index bc3d88b35..b86d9fb8a 100644
--- a/src/Unzer.php
+++ b/src/Unzer.php
@@ -95,7 +95,7 @@ class Unzer implements
      * Construct a new Unzer object.
      *
      * @param string $key The private key your received from your Unzer contact person.
-     * @param ?string $locale The locale of the customer defining defining the translation (e.g. 'en-GB' or 'de-DE').
+     * @param string|null $locale The locale of the customer defining defining the translation (e.g. 'en-GB' or 'de-DE').
      *
      * @throws RuntimeException A RuntimeException will be thrown if the key is not of type private.
      *
@@ -358,7 +358,7 @@ public function getUri(bool $appendId = true, string $httpMethod = HttpAdapterIn
     /**
      * {@inheritDoc}
      */
-    public function activateRecurringPayment($paymentType, string $returnUrl, string $recurrenceType = null): Recurring
+    public function activateRecurringPayment($paymentType, string $returnUrl, ?string $recurrenceType = null): Recurring
     {
         return $this->resourceService->activateRecurringPayment($paymentType, $returnUrl, $recurrenceType);
     }
@@ -667,7 +667,7 @@ public function registerMultipleWebhooks(string $url, array $events): array
     /**
      * {@inheritDoc}
      */
-    public function fetchResourceFromEvent(string $eventJson = null): AbstractUnzerResource
+    public function fetchResourceFromEvent(?string $eventJson = null): AbstractUnzerResource
     {
         return $this->webhookService->fetchResourceFromEvent($eventJson);
     }
@@ -679,8 +679,8 @@ public function performAuthorization(
         Authorization $authorization,
         $paymentType,
         $customer = null,
-        Metadata      $metadata = null,
-        Basket        $basket = null
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null
     ): Authorization
     {
         return $this->paymentService->performAuthorization($authorization, $paymentType, $customer, $metadata, $basket);
@@ -735,8 +735,8 @@ public function performCharge(
         Charge   $charge,
         $paymentType,
         $customer = null,
-        Metadata $metadata = null,
-        Basket   $basket = null
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null
     ): Charge
     {
         return $this->paymentService->performCharge($charge, $paymentType, $customer, $metadata, $basket);
@@ -786,9 +786,9 @@ public function charge(
      */
     public function chargeAuthorization(
         $payment,
-        float $amount = null,
-        string $orderId = null,
-        string $invoiceId = null
+        ?float $amount = null,
+        ?string $orderId = null,
+        ?string $invoiceId = null
     ): Charge
     {
         return $this->paymentService->chargeAuthorization($payment, $amount, $orderId, $invoiceId);
@@ -799,9 +799,9 @@ public function chargeAuthorization(
      */
     public function chargePayment(
         $payment,
-        float $amount = null,
-        string $orderId = null,
-        string $invoiceId = null
+        ?float $amount = null,
+        ?string $orderId = null,
+        ?string $invoiceId = null
     ): Charge
     {
         return $this->paymentService->chargePayment($payment, $amount, $orderId, $invoiceId);
@@ -815,7 +815,7 @@ public function performChargeOnPayment($payment, Charge $charge): Charge
     /**
      * {@inheritDoc}
      */
-    public function cancelAuthorization(Authorization $authorization, float $amount = null): Cancellation
+    public function cancelAuthorization(Authorization $authorization, ?float $amount = null): Cancellation
     {
         return $this->cancelService->cancelAuthorization($authorization, $amount);
     }
@@ -823,7 +823,7 @@ public function cancelAuthorization(Authorization $authorization, float $amount
     /**
      * {@inheritDoc}
      */
-    public function cancelAuthorizationByPayment($payment, float $amount = null): Cancellation
+    public function cancelAuthorizationByPayment($payment, ?float $amount = null): Cancellation
     {
         return $this->cancelService->cancelAuthorizationByPayment($payment, $amount);
     }
@@ -833,11 +833,11 @@ public function cancelAuthorizationByPayment($payment, float $amount = null): Ca
      */
     public function cancelPayment(
         $payment,
-        float $amount = null,
+        ?float $amount = null,
         ?string $reasonCode = CancelReasonCodes::REASON_CODE_CANCEL,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): array
     {
         return $this->cancelService
@@ -847,7 +847,7 @@ public function cancelPayment(
     /**
      * {@inheritDoc}
      */
-    public function cancelPaymentAuthorization($payment, float $amount = null): ?Cancellation
+    public function cancelPaymentAuthorization($payment, ?float $amount = null): ?Cancellation
     {
         return $this->cancelService->cancelPaymentAuthorization($payment, $amount);
     }
@@ -858,11 +858,11 @@ public function cancelPaymentAuthorization($payment, float $amount = null): ?Can
     public function cancelChargeById(
         $payment,
         string $chargeId,
-        float $amount = null,
-        string $reasonCode = null,
-        string $referenceText = null,
-        float $amountNet = null,
-        float $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation
     {
         return $this->cancelService
@@ -874,11 +874,11 @@ public function cancelChargeById(
      */
     public function cancelCharge(
         Charge $charge,
-        float  $amount = null,
-        string $reasonCode = null,
-        string $referenceText = null,
-        float  $amountNet = null,
-        float  $amountVat = null
+        ?float $amount = null,
+        ?string $reasonCode = null,
+        ?string $referenceText = null,
+        ?float $amountNet = null,
+        ?float $amountVat = null
     ): Cancellation
     {
         return $this->cancelService
@@ -917,7 +917,7 @@ public function createAuthToken(): Token
     /**
      * {@inheritDoc}
      */
-    public function ship($payment, string $invoiceId = null, string $orderId = null): Shipment
+    public function ship($payment, ?string $invoiceId = null, ?string $orderId = null): Shipment
     {
         return $this->paymentService->ship($payment, $invoiceId, $orderId);
     }
@@ -931,11 +931,11 @@ public function payout(
         $paymentType,
         string   $returnUrl,
         $customer = null,
-        string   $orderId = null,
-        Metadata $metadata = null,
-        Basket   $basket = null,
-        string   $invoiceId = null,
-        string   $referenceText = null
+        ?string   $orderId = null,
+        ?Metadata $metadata = null,
+        ?Basket   $basket = null,
+        ?string   $invoiceId = null,
+        ?string   $referenceText = null
     ): Payout
     {
         return $this->paymentService->payout(
@@ -1004,9 +1004,9 @@ public function fetchPaypageV2($paypage): PaypageV2
      */
     public function initPayPageCharge(
         Paypage  $paypage,
-        Customer $customer = null,
-        Basket   $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage
     {
         return $this->paymentService->initPayPageCharge($paypage, $customer, $basket, $metadata);
@@ -1017,9 +1017,9 @@ public function initPayPageCharge(
      */
     public function initPayPageAuthorize(
         Paypage  $paypage,
-        Customer $customer = null,
-        Basket   $basket = null,
-        Metadata $metadata = null
+        ?Customer $customer = null,
+        ?Basket   $basket = null,
+        ?Metadata $metadata = null
     ): Paypage
     {
         return $this->paymentService->initPayPageAuthorize($paypage, $customer, $basket, $metadata);
@@ -1032,7 +1032,7 @@ public function fetchInstallmentPlans(
         float    $amount,
         string   $currency,
         float    $effectiveInterest,
-        DateTime $orderDate = null
+        ?DateTime $orderDate = null
     ): InstalmentPlans
     {
         return $this->paymentService
@@ -1078,7 +1078,7 @@ public function debugLog($message): void
      * @param string|null $jwtToken If set, the given token will be used instead as long it is valid.
      * @throws UnzerApiException
      */
-    public function prepareJwtToken(string $jwtToken = null): string
+    public function prepareJwtToken(?string $jwtToken = null): string
     {
         if ($jwtToken !== null && JwtService::validateExpiryTime($jwtToken)) {
             $this->jwtToken = $jwtToken;
diff --git a/test/unit/Services/DummyAdapter.php b/test/unit/Services/DummyAdapter.php
index 399d50cd8..a37787088 100755
--- a/test/unit/Services/DummyAdapter.php
+++ b/test/unit/Services/DummyAdapter.php
@@ -18,7 +18,7 @@ class DummyAdapter implements HttpAdapterInterface
     /**
      * {@inheritDoc}
      */
-    public function init(string $url, string $payload = null, string $httpMethod = HttpAdapterInterface::REQUEST_GET): void
+    public function init(string $url, ?string $payload = null, string $httpMethod = HttpAdapterInterface::REQUEST_GET): void
     {
         // do nothing
     }