diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4a30b839..9e72aa19 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+- Support for PHP `8.4`
+
 ## 5.11.0 - 2024-12-01
 
 ### Added
diff --git a/composer.json b/composer.json
index 90bc86c4..f1c118b8 100644
--- a/composer.json
+++ b/composer.json
@@ -30,7 +30,7 @@
         }
     },
     "require-dev": {
-        "vimeo/psalm": "~5.6",
+        "vimeo/psalm": "~5.6|dev-master",
         "innmind/black-box": "^5.5.1",
         "innmind/coding-standard": "~2.0"
     },
diff --git a/fixtures/Sequence.php b/fixtures/Sequence.php
index 849fc905..d957d00d 100644
--- a/fixtures/Sequence.php
+++ b/fixtures/Sequence.php
@@ -18,7 +18,7 @@ final class Sequence
      *
      * @return Set<Structure<I>>
      */
-    public static function of(Set $set, Set\Integers $sizes = null): Set
+    public static function of(Set $set, ?Set\Integers $sizes = null): Set
     {
         // this is not optimal but it allows to avoid a BC break
         $sizes ??= Set\Integers::between(0, 100);
diff --git a/fixtures/Set.php b/fixtures/Set.php
index b367bb76..b96f8437 100644
--- a/fixtures/Set.php
+++ b/fixtures/Set.php
@@ -21,7 +21,7 @@ final class Set
      *
      * @return Set<Structure<I>>
      */
-    public static function of(DataSet $set, DataSet\Integers $sizes = null): DataSet
+    public static function of(DataSet $set, ?DataSet\Integers $sizes = null): DataSet
     {
         // this is not optimal but it allows to avoid a BC break
         $sizes ??= DataSet\Integers::between(0, 100);
diff --git a/src/Accumulate.php b/src/Accumulate.php
index 89a650f3..7e0c842b 100644
--- a/src/Accumulate.php
+++ b/src/Accumulate.php
@@ -38,6 +38,7 @@ public function current(): mixed
         /** @psalm-suppress UnusedMethodCall */
         $this->pop();
 
+        /** @var S */
         return \current($this->values);
     }
 
diff --git a/src/Map/DoubleIndex.php b/src/Map/DoubleIndex.php
index 52e9516e..41ae1031 100644
--- a/src/Map/DoubleIndex.php
+++ b/src/Map/DoubleIndex.php
@@ -26,7 +26,7 @@ final class DoubleIndex implements Implementation
     /**
      * @param Sequence\Implementation<Pair<T, S>> $pairs
      */
-    public function __construct(Sequence\Implementation $pairs = null)
+    public function __construct(?Sequence\Implementation $pairs = null)
     {
         /** @var Sequence\Implementation<Pair<T, S>> */
         $this->pairs = $pairs ?? new Sequence\Primitive;
diff --git a/src/Map/ObjectKeys.php b/src/Map/ObjectKeys.php
index 78749e15..98bd403a 100644
--- a/src/Map/ObjectKeys.php
+++ b/src/Map/ObjectKeys.php
@@ -22,7 +22,7 @@ final class ObjectKeys implements Implementation
 {
     private \SplObjectStorage $values;
 
-    public function __construct(\SplObjectStorage $values = null)
+    public function __construct(?\SplObjectStorage $values = null)
     {
         $this->values = $values ?? new \SplObjectStorage;
     }
diff --git a/src/Monoid/Append.php b/src/Monoid/Append.php
index ed87dc00..2bc2ea3b 100644
--- a/src/Monoid/Append.php
+++ b/src/Monoid/Append.php
@@ -22,7 +22,7 @@ final class Append implements Monoid
      *
      * @return self<C>
      */
-    public static function of(string $class = null): self
+    public static function of(?string $class = null): self
     {
         /** @var self<C> */
         return new self;
diff --git a/src/Monoid/MergeMap.php b/src/Monoid/MergeMap.php
index 8a7db5c0..1fca56ad 100644
--- a/src/Monoid/MergeMap.php
+++ b/src/Monoid/MergeMap.php
@@ -25,7 +25,7 @@ final class MergeMap implements Monoid
      *
      * @return self<A, B>
      */
-    public static function of(string $key = null, string $value = null): self
+    public static function of(?string $key = null, ?string $value = null): self
     {
         /** @var self<A, B> */
         return new self;
diff --git a/src/Monoid/MergeSet.php b/src/Monoid/MergeSet.php
index 25cc16a3..a5e2f53f 100644
--- a/src/Monoid/MergeSet.php
+++ b/src/Monoid/MergeSet.php
@@ -22,7 +22,7 @@ final class MergeSet implements Monoid
      *
      * @return self<C>
      */
-    public static function of(string $class = null): self
+    public static function of(?string $class = null): self
     {
         /** @var self<C> */
         return new self;
diff --git a/src/Sequence/Aggregate.php b/src/Sequence/Aggregate.php
index 1e378ca9..e0198899 100644
--- a/src/Sequence/Aggregate.php
+++ b/src/Sequence/Aggregate.php
@@ -58,7 +58,10 @@ public function __invoke(callable $map): \Generator
 
         /** @psalm-suppress ImpureMethodCall */
         while ($this->values->valid()) {
-            /** @psalm-suppress ImpureFunctionCall */
+            /**
+             * @psalm-suppress PossiblyNullArgument
+             * @psalm-suppress ImpureFunctionCall
+             */
             $aggregate = $this->walk($map($n2, $n1), $void);
 
             foreach ($aggregate as $element) {
diff --git a/src/Sequence/Lazy.php b/src/Sequence/Lazy.php
index fcf8ac38..5f8731a8 100644
--- a/src/Sequence/Lazy.php
+++ b/src/Sequence/Lazy.php
@@ -860,6 +860,7 @@ public function dropWhile(callable $condition): self
                 /**
                  * @psalm-suppress ImpureMethodCall
                  * @psalm-suppress ImpureFunctionCall
+                 * @psalm-suppress PossiblyNullArgument
                  */
                 if (!$condition($generator->current())) {
                     /** @psalm-suppress ImpureMethodCall */
diff --git a/src/Sequence/Primitive.php b/src/Sequence/Primitive.php
index 644fa5e1..6e5e1734 100644
--- a/src/Sequence/Primitive.php
+++ b/src/Sequence/Primitive.php
@@ -609,6 +609,7 @@ public function dropWhile(callable $condition): self
             /**
              * @psalm-suppress ImpureMethodCall
              * @psalm-suppress ImpureFunctionCall
+             * @psalm-suppress PossiblyNullArgument
              */
             if (!$condition($iterator->current())) {
                 /** @psalm-suppress ImpureMethodCall */
diff --git a/src/Str.php b/src/Str.php
index 3a169f1d..c861fb6e 100644
--- a/src/Str.php
+++ b/src/Str.php
@@ -13,7 +13,7 @@ final class Str implements \Stringable
     private string $value;
     private Str\Encoding $encoding;
 
-    private function __construct(string $value, Str\Encoding $encoding = null)
+    private function __construct(string $value, ?Str\Encoding $encoding = null)
     {
         $this->value = $value;
         $this->encoding = $encoding ?? Str\Encoding::utf8;
@@ -27,7 +27,7 @@ public function __toString(): string
     /**
      * @psalm-pure
      */
-    public static function of(string $value, Str\Encoding $encoding = null): self
+    public static function of(string $value, ?Str\Encoding $encoding = null): self
     {
         return new self($value, $encoding);
     }
@@ -75,7 +75,7 @@ public function maybe(callable $filter): Maybe
      *
      * @return Sequence<self>
      */
-    public function split(string $delimiter = null): Sequence
+    public function split(?string $delimiter = null): Sequence
     {
         if (\is_null($delimiter) || $delimiter === '') {
             return $this->chunk();
@@ -279,12 +279,20 @@ public function words(string|\Stringable $charlist = ''): Map
     /**
      * Split the string using a regular expression
      *
+     * @throws InvalidRegex If the split didn't work
+     *
      * @return Sequence<self>
      */
     public function pregSplit(string|\Stringable $regex, int $limit = -1): Sequence
     {
         /** @psalm-suppress ArgumentTypeCoercion */
         $strings = \preg_split((string) $regex, $this->value, $limit);
+
+        if ($strings === false) {
+            /** @psalm-suppress ImpureFunctionCall */
+            throw new InvalidRegex('', \preg_last_error());
+        }
+
         /** @var Sequence<self> */
         $sequence = Sequence::of();
 
@@ -348,7 +356,7 @@ public function pregReplace(
      *
      * @param 0|positive-int $length
      */
-    public function substring(int $start, int $length = null): self
+    public function substring(int $start, ?int $length = null): self
     {
         if ($this->empty()) {
             return $this;
@@ -462,7 +470,7 @@ public function equals(self $string): bool
     /**
      * Trim the string
      */
-    public function trim(string $mask = null): self
+    public function trim(?string $mask = null): self
     {
         return new self(
             $mask === null ? \trim($this->value) : \trim($this->value, $mask),
@@ -473,7 +481,7 @@ public function trim(string $mask = null): self
     /**
      * Trim the right side of the string
      */
-    public function rightTrim(string $mask = null): self
+    public function rightTrim(?string $mask = null): self
     {
         return new self(
             $mask === null ? \rtrim($this->value) : \rtrim($this->value, $mask),
@@ -484,7 +492,7 @@ public function rightTrim(string $mask = null): self
     /**
      * Trim the left side of the string
      */
-    public function leftTrim(string $mask = null): self
+    public function leftTrim(?string $mask = null): self
     {
         return new self(
             $mask === null ? \ltrim($this->value) : \ltrim($this->value, $mask),