diff --git a/src/Functional/CompareObjectHashOn.php b/src/Functional/CompareObjectHashOn.php
index c4194479..35e0ce04 100644
--- a/src/Functional/CompareObjectHashOn.php
+++ b/src/Functional/CompareObjectHashOn.php
@@ -14,11 +14,11 @@
  * Returns a comparison function that can be used with e.g. `usort()`
  *
  * @param callable $comparison A function that compares the two values. Pick e.g. strcmp() or strnatcasecmp()
- * @param callable $keyFunction A function that takes an argument and returns the value that should be compared
+ * @param callable|null $keyFunction A function that takes an argument and returns the value that should be compared
  * @return callable
  * @no-named-arguments
  */
-function compare_object_hash_on(callable $comparison, callable $keyFunction = null)
+function compare_object_hash_on(callable $comparison, ?callable $keyFunction = null)
 {
     $keyFunction = $keyFunction ? compose($keyFunction, 'spl_object_hash') : 'spl_object_hash';
 
diff --git a/src/Functional/CompareOn.php b/src/Functional/CompareOn.php
index b0e5cda0..2d7f2486 100644
--- a/src/Functional/CompareOn.php
+++ b/src/Functional/CompareOn.php
@@ -14,11 +14,11 @@
  * Returns a comparison function that can be used with e.g. `usort()`
  *
  * @param callable $comparison A function that compares the two values. Pick e.g. strcmp() or strnatcasecmp()
- * @param callable $reducer A function that takes an argument and returns the value that should be compared
+ * @param callable|null $reducer A function that takes an argument and returns the value that should be compared
  * @return callable
  * @no-named-arguments
  */
-function compare_on(callable $comparison, callable $reducer = null)
+function compare_on(callable $comparison, ?callable $reducer = null)
 {
     if ($reducer === null) {
         return static function ($left, $right) use ($comparison) {
diff --git a/src/Functional/Every.php b/src/Functional/Every.php
index e4d8ff62..667cd72b 100644
--- a/src/Functional/Every.php
+++ b/src/Functional/Every.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function every($collection, callable $callback = null)
+function every($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/First.php b/src/Functional/First.php
index 87db945e..20c39274 100644
--- a/src/Functional/First.php
+++ b/src/Functional/First.php
@@ -19,11 +19,11 @@
  * arguments will be element, index, collection
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return mixed
  * @no-named-arguments
  */
-function first($collection, callable $callback = null)
+function first($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Group.php b/src/Functional/Group.php
index af2ed3a1..0a533d44 100644
--- a/src/Functional/Group.php
+++ b/src/Functional/Group.php
@@ -32,6 +32,11 @@ function group($collection, callable $callback)
 
         InvalidArgumentException::assertValidArrayKey($groupKey, __FUNCTION__);
 
+        // Avoid implicit precision-loss from doubles (which cannot be keys)
+        if (is_numeric($groupKey)) {
+            $groupKey = intval($groupKey);
+        }
+
         if (!isset($groups[$groupKey])) {
             $groups[$groupKey] = [];
         }
diff --git a/src/Functional/Head.php b/src/Functional/Head.php
index c6de5e86..3019ee86 100644
--- a/src/Functional/Head.php
+++ b/src/Functional/Head.php
@@ -17,11 +17,11 @@
  * Alias for Functional\first
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return mixed
  * @no-named-arguments
  */
-function head($collection, callable $callback = null)
+function head($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Last.php b/src/Functional/Last.php
index 8989f945..3c861d31 100644
--- a/src/Functional/Last.php
+++ b/src/Functional/Last.php
@@ -18,11 +18,11 @@
  * Callback arguments will be element, index, collection
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return mixed
  * @no-named-arguments
  */
-function last($collection, callable $callback = null)
+function last($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Memoize.php b/src/Functional/Memoize.php
index 3813e3ce..aca716b5 100644
--- a/src/Functional/Memoize.php
+++ b/src/Functional/Memoize.php
@@ -21,7 +21,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function memoize(callable $callback = null, $arguments = [], $key = null)
+function memoize(?callable $callback = null, $arguments = [], $key = null)
 {
     static $storage = [];
     if ($callback === null) {
diff --git a/src/Functional/None.php b/src/Functional/None.php
index edbd33a9..c90e4055 100644
--- a/src/Functional/None.php
+++ b/src/Functional/None.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function none($collection, callable $callback = null)
+function none($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Pick.php b/src/Functional/Pick.php
index b4be3c13..7d29f92c 100644
--- a/src/Functional/Pick.php
+++ b/src/Functional/Pick.php
@@ -24,7 +24,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function pick($collection, $index, $default = null, callable $callback = null)
+function pick($collection, $index, $default = null, ?callable $callback = null)
 {
     InvalidArgumentException::assertArrayAccess($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Poll.php b/src/Functional/Poll.php
index 7c632d77..8f920d24 100644
--- a/src/Functional/Poll.php
+++ b/src/Functional/Poll.php
@@ -26,7 +26,7 @@
  * @return boolean
  * @no-named-arguments
  */
-function poll(callable $callback, $timeout, Traversable $delaySequence = null)
+function poll(callable $callback, $timeout, ?Traversable $delaySequence = null)
 {
     InvalidArgumentException::assertIntegerGreaterThanOrEqual($timeout, 0, __FUNCTION__, 2);
 
diff --git a/src/Functional/Reject.php b/src/Functional/Reject.php
index 505acc91..0efddeee 100644
--- a/src/Functional/Reject.php
+++ b/src/Functional/Reject.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function reject($collection, callable $callback = null)
+function reject($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Retry.php b/src/Functional/Retry.php
index 40ef0fa7..4466542a 100644
--- a/src/Functional/Retry.php
+++ b/src/Functional/Retry.php
@@ -29,7 +29,7 @@
  * @return mixed Return value of the function
  * @no-named-arguments
  */
-function retry(callable $callback, $retries, Traversable $delaySequence = null)
+function retry(callable $callback, $retries, ?Traversable $delaySequence = null)
 {
     InvalidArgumentException::assertIntegerGreaterThanOrEqual($retries, 1, __FUNCTION__, 2);
 
diff --git a/src/Functional/Select.php b/src/Functional/Select.php
index 93420357..eb0ab73d 100644
--- a/src/Functional/Select.php
+++ b/src/Functional/Select.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function select($collection, callable $callback = null)
+function select($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Sequences/ExponentialSequence.php b/src/Functional/Sequences/ExponentialSequence.php
index 0a8b8d47..b550545a 100644
--- a/src/Functional/Sequences/ExponentialSequence.php
+++ b/src/Functional/Sequences/ExponentialSequence.php
@@ -38,28 +38,28 @@ public function __construct($start, $percentage)
         $this->percentage = $percentage;
     }
 
-    public function current()
+    public function current(): int
     {
         return $this->value;
     }
 
-    public function next()
+    public function next(): void
     {
         $this->value = (int) \round(\pow($this->start * (1 + $this->percentage / 100), $this->times));
         $this->times++;
     }
 
-    public function key()
+    public function key(): ?int
     {
         return null;
     }
 
-    public function valid()
+    public function valid(): bool
     {
         return true;
     }
 
-    public function rewind()
+    public function rewind(): void
     {
         $this->times = 1;
         $this->value = $this->start;
diff --git a/src/Functional/Sequences/LinearSequence.php b/src/Functional/Sequences/LinearSequence.php
index ad99dfbb..e7345a6a 100644
--- a/src/Functional/Sequences/LinearSequence.php
+++ b/src/Functional/Sequences/LinearSequence.php
@@ -34,27 +34,27 @@ public function __construct($start, $amount)
         $this->amount = $amount;
     }
 
-    public function current()
+    public function current(): int
     {
         return $this->value;
     }
 
-    public function next()
+    public function next(): void
     {
         $this->value += $this->amount;
     }
 
-    public function key()
+    public function key(): int
     {
         return 0;
     }
 
-    public function valid()
+    public function valid(): bool
     {
         return true;
     }
 
-    public function rewind()
+    public function rewind(): void
     {
         $this->value = $this->start;
     }
diff --git a/src/Functional/Some.php b/src/Functional/Some.php
index df27e0a8..9ae81ade 100644
--- a/src/Functional/Some.php
+++ b/src/Functional/Some.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function some($collection, callable $callback = null)
+function some($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Tail.php b/src/Functional/Tail.php
index 495131d2..33637ffe 100644
--- a/src/Functional/Tail.php
+++ b/src/Functional/Tail.php
@@ -18,11 +18,11 @@
  * Takes an optional callback for filtering the collection.
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return array
  * @no-named-arguments
  */
-function tail($collection, callable $callback = null)
+function tail($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Unique.php b/src/Functional/Unique.php
index 656ac2be..cfa12442 100644
--- a/src/Functional/Unique.php
+++ b/src/Functional/Unique.php
@@ -17,12 +17,12 @@
  * Returns an array of unique elements
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @param bool $strict
  * @return array
  * @no-named-arguments
  */
-function unique($collection, callable $callback = null, $strict = true)
+function unique($collection, ?callable $callback = null, $strict = true)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/tests/Functional/ErrorToExceptionTest.php b/tests/Functional/ErrorToExceptionTest.php
index e68893e0..f5280cd8 100644
--- a/tests/Functional/ErrorToExceptionTest.php
+++ b/tests/Functional/ErrorToExceptionTest.php
@@ -21,6 +21,14 @@ class ErrorToExceptionTest extends AbstractTestCase
 {
     public function testErrorIsThrownAsException(): void
     {
+        if (PHP_VERSION_ID >= 80401) {
+            // As of PHP 8.4, trigger_error('...', E_USER_ERROR) is deprecated
+            // This means the actual error thrown is different from the one
+            // attempted
+            // See https://3v4l.org/MmFA6.
+            self::markTestSkipped('Only works with PHP <8.4 due to deprecated E_USER_ERROR');
+        }
+
         $origFn = function () {
             \trigger_error('Some error', E_USER_ERROR);
         };
diff --git a/tests/Functional/RepeatTest.php b/tests/Functional/RepeatTest.php
index 9ab5e8a4..b58ee698 100644
--- a/tests/Functional/RepeatTest.php
+++ b/tests/Functional/RepeatTest.php
@@ -45,8 +45,13 @@ public function test(): void
     public function testNegativeRepeatedTimes(): void
     {
         $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage(
-            'Functional\{closure}() expects parameter 1 to be positive integer, negative integer given'
+
+        // See https://3v4l.org/Ms79G for message formats
+        // See https://regex101.com/r/hTvW3o/1 for regex setup
+        $this->expectExceptionMessageMatches(
+            '/(Functional\\\\{closure}' // PHP < 8.4
+                . '|{closure:Functional\\\\repeat\(\):[0-9]+})' // PHP 8.4+
+                . '\(\) expects parameter 1 to be positive integer, negative integer given/'
         );
 
         repeat([$this->repeated, 'foo'])(-1);