diff --git a/ExpressionFunction.php b/ExpressionFunction.php
index d0ddd10..0b3d6c4 100644
--- a/ExpressionFunction.php
+++ b/ExpressionFunction.php
@@ -70,7 +70,7 @@ public function getEvaluator(): \Closure
      * @throws \InvalidArgumentException if given PHP function name is in namespace
      *                                   and expression function name is not defined
      */
-    public static function fromPhp(string $phpFunctionName, string $expressionFunctionName = null): self
+    public static function fromPhp(string $phpFunctionName, ?string $expressionFunctionName = null): self
     {
         $phpFunctionName = ltrim($phpFunctionName, '\\');
         if (!\function_exists($phpFunctionName)) {
diff --git a/ExpressionLanguage.php b/ExpressionLanguage.php
index 9e10740..a7f249f 100644
--- a/ExpressionLanguage.php
+++ b/ExpressionLanguage.php
@@ -34,7 +34,7 @@ class ExpressionLanguage
     /**
      * @param ExpressionFunctionProviderInterface[] $providers
      */
-    public function __construct(CacheItemPoolInterface $cache = null, array $providers = [])
+    public function __construct(?CacheItemPoolInterface $cache = null, array $providers = [])
     {
         $this->cache = $cache ?? new ArrayAdapter();
         $this->registerFunctions();
diff --git a/Node/ArrayNode.php b/Node/ArrayNode.php
index 993af36..79eade2 100644
--- a/Node/ArrayNode.php
+++ b/Node/ArrayNode.php
@@ -27,7 +27,7 @@ public function __construct()
         $this->index = -1;
     }
 
-    public function addElement(Node $value, Node $key = null): void
+    public function addElement(Node $value, ?Node $key = null): void
     {
         $key ??= new ConstantNode(++$this->index);
 
diff --git a/SyntaxError.php b/SyntaxError.php
index 0bfd7e9..e165dc2 100644
--- a/SyntaxError.php
+++ b/SyntaxError.php
@@ -13,7 +13,7 @@
 
 class SyntaxError extends \LogicException
 {
-    public function __construct(string $message, int $cursor = 0, string $expression = '', string $subject = null, array $proposals = null)
+    public function __construct(string $message, int $cursor = 0, string $expression = '', ?string $subject = null, ?array $proposals = null)
     {
         $message = sprintf('%s around position %d', rtrim($message, '.'), $cursor);
         if ($expression) {
diff --git a/Tests/ExpressionLanguageTest.php b/Tests/ExpressionLanguageTest.php
index 167f784..93dbeb6 100644
--- a/Tests/ExpressionLanguageTest.php
+++ b/Tests/ExpressionLanguageTest.php
@@ -366,7 +366,7 @@ public function testNullSafeCompileFails($expression, $foo)
 
         $this->expectException(\ErrorException::class);
 
-        set_error_handler(static function (int $errno, string $errstr, string $errfile = null, int $errline = null): bool {
+        set_error_handler(static function (int $errno, string $errstr, ?string $errfile = null, ?int $errline = null): bool {
             if ($errno & (\E_WARNING | \E_USER_WARNING) && (str_contains($errstr, 'Attempt to read property') || str_contains($errstr, 'Trying to access'))) {
                 throw new \ErrorException($errstr, 0, $errno, $errfile, $errline);
             }
diff --git a/Tests/ParserTest.php b/Tests/ParserTest.php
index 58a232e..9c546e3 100644
--- a/Tests/ParserTest.php
+++ b/Tests/ParserTest.php
@@ -284,7 +284,7 @@ public function testNameProposal()
     /**
      * @dataProvider getLintData
      */
-    public function testLint($expression, $names, string $exception = null)
+    public function testLint($expression, $names, ?string $exception = null)
     {
         if ($exception) {
             $this->expectException(SyntaxError::class);
diff --git a/Token.php b/Token.php
index 6eff31e..99f721f 100644
--- a/Token.php
+++ b/Token.php
@@ -51,7 +51,7 @@ public function __toString(): string
     /**
      * Tests the current token for a type and/or a value.
      */
-    public function test(string $type, string $value = null): bool
+    public function test(string $type, ?string $value = null): bool
     {
         return $this->type === $type && (null === $value || $this->value == $value);
     }
diff --git a/TokenStream.php b/TokenStream.php
index 241725b..9512a10 100644
--- a/TokenStream.php
+++ b/TokenStream.php
@@ -60,7 +60,7 @@ public function next()
      *
      * @return void
      */
-    public function expect(string $type, string $value = null, string $message = null)
+    public function expect(string $type, ?string $value = null, ?string $message = null)
     {
         $token = $this->current;
         if (!$token->test($type, $value)) {