diff --git a/src/AuthorizationHeaderBuilder.php b/src/AuthorizationHeaderBuilder.php
index ab105f0..7bedc15 100644
--- a/src/AuthorizationHeaderBuilder.php
+++ b/src/AuthorizationHeaderBuilder.php
@@ -77,7 +77,7 @@ class AuthorizationHeaderBuilder
      *   The message digest to use when signing requests. Defaults to
      *   \Acquia\Hmac\Digest\Digest.
      */
-    public function __construct(RequestInterface $request, KeyInterface $key, DigestInterface $digest = null)
+    public function __construct(RequestInterface $request, KeyInterface $key, ?DigestInterface $digest = null)
     {
         $this->request = $request;
         $this->key     = $key;
diff --git a/src/RequestSigner.php b/src/RequestSigner.php
index 3452c07..09b1047 100644
--- a/src/RequestSigner.php
+++ b/src/RequestSigner.php
@@ -62,7 +62,7 @@ public function signRequest(RequestInterface $request, array $customHeaders = []
     /**
      * {@inheritDoc}
      */
-    public function getTimestampedRequest(RequestInterface $request, \DateTime $date = null)
+    public function getTimestampedRequest(RequestInterface $request, ?\DateTime $date = null)
     {
         if ($request->hasHeader('X-Authorization-Timestamp')) {
             return clone $request;
diff --git a/src/ResponseSigner.php b/src/ResponseSigner.php
index 017e392..7b7a879 100644
--- a/src/ResponseSigner.php
+++ b/src/ResponseSigner.php
@@ -40,7 +40,7 @@ class ResponseSigner implements ResponseSignerInterface
      *   The digest with which to sign the response. Defaults to
      *   \Acquia\Hmac\Digest\Digest.
      */
-    public function __construct(KeyInterface $key, RequestInterface $request, DigestInterface $digest = null)
+    public function __construct(KeyInterface $key, RequestInterface $request, ?DigestInterface $digest = null)
     {
         $this->key = $key;
         $this->request = $request;