diff --git a/AbstractString.php b/AbstractString.php
index ed1417a..6b125d4 100644
--- a/AbstractString.php
+++ b/AbstractString.php
@@ -383,7 +383,7 @@ public function isEmpty(): bool
         return '' === $this->string;
     }
 
-    abstract public function join(array $strings, string $lastGlue = null): static;
+    abstract public function join(array $strings, ?string $lastGlue = null): static;
 
     public function jsonSerialize(): string
     {
@@ -429,16 +429,16 @@ abstract public function replaceMatches(string $fromRegexp, string|callable $to)
 
     abstract public function reverse(): static;
 
-    abstract public function slice(int $start = 0, int $length = null): static;
+    abstract public function slice(int $start = 0, ?int $length = null): static;
 
     abstract public function snake(): static;
 
-    abstract public function splice(string $replacement, int $start = 0, int $length = null): static;
+    abstract public function splice(string $replacement, int $start = 0, ?int $length = null): static;
 
     /**
      * @return static[]
      */
-    public function split(string $delimiter, int $limit = null, int $flags = null): array
+    public function split(string $delimiter, ?int $limit = null, ?int $flags = null): array
     {
         if (null === $flags) {
             throw new \TypeError('Split behavior when $flags is null must be implemented by child classes.');
@@ -495,7 +495,7 @@ public function startsWith(string|iterable $prefix): bool
 
     abstract public function title(bool $allWords = false): static;
 
-    public function toByteString(string $toEncoding = null): ByteString
+    public function toByteString(?string $toEncoding = null): ByteString
     {
         $b = new ByteString();
 
diff --git a/AbstractUnicodeString.php b/AbstractUnicodeString.php
index d19a61a..74c47a0 100644
--- a/AbstractUnicodeString.php
+++ b/AbstractUnicodeString.php
@@ -198,7 +198,7 @@ public function folded(bool $compat = true): static
         return $str;
     }
 
-    public function join(array $strings, string $lastGlue = null): static
+    public function join(array $strings, ?string $lastGlue = null): static
     {
         $str = clone $this;
 
diff --git a/ByteString.php b/ByteString.php
index 212290f..8ef3981 100644
--- a/ByteString.php
+++ b/ByteString.php
@@ -42,7 +42,7 @@ public function __construct(string $string = '')
      * Copyright (c) 2004-2020, Facebook, Inc. (https://www.facebook.com/)
      */
 
-    public static function fromRandom(int $length = 16, string $alphabet = null): self
+    public static function fromRandom(int $length = 16, ?string $alphabet = null): self
     {
         if ($length <= 0) {
             throw new InvalidArgumentException(sprintf('A strictly positive length is expected, "%d" given.', $length));
@@ -205,7 +205,7 @@ public function isUtf8(): bool
         return '' === $this->string || preg_match('//u', $this->string);
     }
 
-    public function join(array $strings, string $lastGlue = null): static
+    public function join(array $strings, ?string $lastGlue = null): static
     {
         $str = clone $this;
 
@@ -332,7 +332,7 @@ public function reverse(): static
         return $str;
     }
 
-    public function slice(int $start = 0, int $length = null): static
+    public function slice(int $start = 0, ?int $length = null): static
     {
         $str = clone $this;
         $str->string = (string) substr($this->string, $start, $length ?? \PHP_INT_MAX);
@@ -348,7 +348,7 @@ public function snake(): static
         return $str;
     }
 
-    public function splice(string $replacement, int $start = 0, int $length = null): static
+    public function splice(string $replacement, int $start = 0, ?int $length = null): static
     {
         $str = clone $this;
         $str->string = substr_replace($this->string, $replacement, $start, $length ?? \PHP_INT_MAX);
@@ -356,7 +356,7 @@ public function splice(string $replacement, int $start = 0, int $length = null):
         return $str;
     }
 
-    public function split(string $delimiter, int $limit = null, int $flags = null): array
+    public function split(string $delimiter, ?int $limit = null, ?int $flags = null): array
     {
         if (1 > $limit ??= \PHP_INT_MAX) {
             throw new InvalidArgumentException('Split limit must be a positive integer.');
@@ -402,12 +402,12 @@ public function title(bool $allWords = false): static
         return $str;
     }
 
-    public function toUnicodeString(string $fromEncoding = null): UnicodeString
+    public function toUnicodeString(?string $fromEncoding = null): UnicodeString
     {
         return new UnicodeString($this->toCodePointString($fromEncoding)->string);
     }
 
-    public function toCodePointString(string $fromEncoding = null): CodePointString
+    public function toCodePointString(?string $fromEncoding = null): CodePointString
     {
         $u = new CodePointString();
 
diff --git a/CodePointString.php b/CodePointString.php
index f5c900f..337bfc1 100644
--- a/CodePointString.php
+++ b/CodePointString.php
@@ -186,7 +186,7 @@ public function replace(string $from, string $to): static
         return $str;
     }
 
-    public function slice(int $start = 0, int $length = null): static
+    public function slice(int $start = 0, ?int $length = null): static
     {
         $str = clone $this;
         $str->string = mb_substr($this->string, $start, $length, 'UTF-8');
@@ -194,7 +194,7 @@ public function slice(int $start = 0, int $length = null): static
         return $str;
     }
 
-    public function splice(string $replacement, int $start = 0, int $length = null): static
+    public function splice(string $replacement, int $start = 0, ?int $length = null): static
     {
         if (!preg_match('//u', $replacement)) {
             throw new InvalidArgumentException('Invalid UTF-8 string.');
@@ -208,7 +208,7 @@ public function splice(string $replacement, int $start = 0, int $length = null):
         return $str;
     }
 
-    public function split(string $delimiter, int $limit = null, int $flags = null): array
+    public function split(string $delimiter, ?int $limit = null, ?int $flags = null): array
     {
         if (1 > $limit ??= \PHP_INT_MAX) {
             throw new InvalidArgumentException('Split limit must be a positive integer.');
diff --git a/Slugger/AsciiSlugger.php b/Slugger/AsciiSlugger.php
index 6e550c6..a9693d4 100644
--- a/Slugger/AsciiSlugger.php
+++ b/Slugger/AsciiSlugger.php
@@ -68,7 +68,7 @@ class AsciiSlugger implements SluggerInterface, LocaleAwareInterface
      */
     private array $transliterators = [];
 
-    public function __construct(string $defaultLocale = null, array|\Closure $symbolsMap = null)
+    public function __construct(?string $defaultLocale = null, array|\Closure|null $symbolsMap = null)
     {
         $this->defaultLocale = $defaultLocale;
         $this->symbolsMap = $symbolsMap ?? $this->symbolsMap;
@@ -104,7 +104,7 @@ public function withEmoji(bool|string $emoji = true): static
         return $new;
     }
 
-    public function slug(string $string, string $separator = '-', string $locale = null): AbstractUnicodeString
+    public function slug(string $string, string $separator = '-', ?string $locale = null): AbstractUnicodeString
     {
         $locale ??= $this->defaultLocale;
 
diff --git a/Slugger/SluggerInterface.php b/Slugger/SluggerInterface.php
index c679ed9..dd0d581 100644
--- a/Slugger/SluggerInterface.php
+++ b/Slugger/SluggerInterface.php
@@ -23,5 +23,5 @@ interface SluggerInterface
     /**
      * Creates a slug for the given string and locale, using appropriate transliteration when needed.
      */
-    public function slug(string $string, string $separator = '-', string $locale = null): AbstractUnicodeString;
+    public function slug(string $string, string $separator = '-', ?string $locale = null): AbstractUnicodeString;
 }
diff --git a/Tests/AbstractAsciiTestCase.php b/Tests/AbstractAsciiTestCase.php
index b4578c7..eabe7b9 100644
--- a/Tests/AbstractAsciiTestCase.php
+++ b/Tests/AbstractAsciiTestCase.php
@@ -46,7 +46,7 @@ public function testCreateFromEmptyString()
     /**
      * @dataProvider provideBytesAt
      */
-    public function testBytesAt(array $expected, string $string, int $offset, int $form = null)
+    public function testBytesAt(array $expected, string $string, int $offset, ?int $form = null)
     {
         if (2 !== grapheme_strlen('च्छे') && 'नमस्ते' === $string) {
             $this->markTestSkipped('Skipping due to issue ICU-21661.');
@@ -319,7 +319,7 @@ public static function provideIndexOfLastIgnoreCase(): array
     /**
      * @dataProvider provideSplit
      */
-    public function testSplit(string $string, string $delimiter, array $chunks, ?int $limit, int $flags = null)
+    public function testSplit(string $string, string $delimiter, array $chunks, ?int $limit, ?int $flags = null)
     {
         $this->assertEquals($chunks, static::createFromString($string)->split($delimiter, $limit, $flags));
     }
@@ -595,7 +595,7 @@ public static function provideTitle()
     /**
      * @dataProvider provideSlice
      */
-    public function testSlice(string $expected, string $origin, int $start, int $length = null)
+    public function testSlice(string $expected, string $origin, int $start, ?int $length = null)
     {
         $this->assertEquals(
             static::createFromString($expected),
@@ -623,7 +623,7 @@ public static function provideSlice()
     /**
      * @dataProvider provideSplice
      */
-    public function testSplice(string $expected, int $start, int $length = null)
+    public function testSplice(string $expected, int $start, ?int $length = null)
     {
         $this->assertEquals(
             static::createFromString($expected),
@@ -1081,7 +1081,7 @@ public static function provideSnake()
     /**
      * @dataProvider provideStartsWith
      */
-    public function testStartsWith(bool $expected, string $origin, $prefix, int $form = null)
+    public function testStartsWith(bool $expected, string $origin, $prefix, ?int $form = null)
     {
         $instance = static::createFromString($origin);
         $instance = $form ? $instance->normalize($form) : $instance;
@@ -1135,7 +1135,7 @@ public static function provideStartsWithIgnoreCase()
     /**
      * @dataProvider provideEndsWith
      */
-    public function testEndsWith(bool $expected, string $origin, $suffix, int $form = null)
+    public function testEndsWith(bool $expected, string $origin, $suffix, ?int $form = null)
     {
         $instance = static::createFromString($origin);
         $instance = $form ? $instance->normalize($form) : $instance;
diff --git a/Tests/AbstractUnicodeTestCase.php b/Tests/AbstractUnicodeTestCase.php
index e95dddd..049d0b1 100644
--- a/Tests/AbstractUnicodeTestCase.php
+++ b/Tests/AbstractUnicodeTestCase.php
@@ -78,7 +78,7 @@ public static function provideBytesAt(): array
     /**
      * @dataProvider provideCodePointsAt
      */
-    public function testCodePointsAt(array $expected, string $string, int $offset, int $form = null)
+    public function testCodePointsAt(array $expected, string $string, int $offset, ?int $form = null)
     {
         if (2 !== grapheme_strlen('च्छे') && 'नमस्ते' === $string) {
             $this->markTestSkipped('Skipping due to issue ICU-21661.');
diff --git a/Tests/Slugger/AsciiSluggerTest.php b/Tests/Slugger/AsciiSluggerTest.php
index 3544367..703212f 100644
--- a/Tests/Slugger/AsciiSluggerTest.php
+++ b/Tests/Slugger/AsciiSluggerTest.php
@@ -19,7 +19,7 @@ class AsciiSluggerTest extends TestCase
     /**
      * @dataProvider provideSlugTests
      */
-    public function testSlug(string $expected, string $string, string $separator = '-', string $locale = null)
+    public function testSlug(string $expected, string $string, string $separator = '-', ?string $locale = null)
     {
         $slugger = new AsciiSlugger();
 
diff --git a/UnicodeString.php b/UnicodeString.php
index a64c6a9..a10208b 100644
--- a/UnicodeString.php
+++ b/UnicodeString.php
@@ -176,7 +176,7 @@ public function indexOfLast(string|iterable|AbstractString $needle, int $offset
         return false === $i ? null : $i;
     }
 
-    public function join(array $strings, string $lastGlue = null): static
+    public function join(array $strings, ?string $lastGlue = null): static
     {
         $str = parent::join($strings, $lastGlue);
         normalizer_is_normalized($str->string) ?: $str->string = normalizer_normalize($str->string);
@@ -253,7 +253,7 @@ public function replaceMatches(string $fromRegexp, string|callable $to): static
         return $str;
     }
 
-    public function slice(int $start = 0, int $length = null): static
+    public function slice(int $start = 0, ?int $length = null): static
     {
         $str = clone $this;
 
@@ -262,7 +262,7 @@ public function slice(int $start = 0, int $length = null): static
         return $str;
     }
 
-    public function splice(string $replacement, int $start = 0, int $length = null): static
+    public function splice(string $replacement, int $start = 0, ?int $length = null): static
     {
         $str = clone $this;
 
@@ -278,7 +278,7 @@ public function splice(string $replacement, int $start = 0, int $length = null):
         return $str;
     }
 
-    public function split(string $delimiter, int $limit = null, int $flags = null): array
+    public function split(string $delimiter, ?int $limit = null, ?int $flags = null): array
     {
         if (1 > $limit ??= 2147483647) {
             throw new InvalidArgumentException('Split limit must be a positive integer.');