diff --git a/.github/workflows/phpunit.yml b/.github/workflows/phpunit.yml
index ad6af891..4c9d3c6c 100644
--- a/.github/workflows/phpunit.yml
+++ b/.github/workflows/phpunit.yml
@@ -24,6 +24,8 @@ jobs:
           - "8.0"
           - "8.1"
           - "8.2"
+          - "8.3"
+          - "8.4"
         operating-system:
           - "ubuntu-latest"
       fail-fast: false
@@ -50,10 +52,13 @@ jobs:
           restore-keys: "php-${{ matrix.php-version }}"
 
       - name: "Test with lowest dependencies"
-        if: "matrix.php-version != '8.2'"
+        if: "matrix.php-version != '8.4'"
         run: |
-          composer update --prefer-lowest --no-interaction --no-progress --no-suggest && vendor/bin/simple-phpunit
+          composer update --prefer-lowest --no-interaction --no-progress --no-suggest
+          composer update symfony/phpunit-bridge --no-interaction --no-progress --no-suggest
+          vendor/bin/simple-phpunit
 
       - name: "Test with highest dependencies"
         run: |
-          composer update --no-interaction --no-progress --no-suggest $([[ "${{ matrix.php-version }}" = "8.2" ]] && echo ' --ignore-platform-req=php+') && vendor/bin/simple-phpunit
+          composer update --no-interaction --no-progress --no-suggest $([[ "${{ matrix.php-version }}" = "8.4" ]] && echo ' --ignore-platform-req=php+')
+          vendor/bin/simple-phpunit
diff --git a/src/ProxyManager/Generator/MethodGenerator.php b/src/ProxyManager/Generator/MethodGenerator.php
index ce4937fd..1349c4e7 100644
--- a/src/ProxyManager/Generator/MethodGenerator.php
+++ b/src/ProxyManager/Generator/MethodGenerator.php
@@ -67,6 +67,11 @@ public static function copyMethodSignature(MethodReflection $reflectionMethod):
 
             if ($default !== null) {
                 $parameter->setDefaultValue(new ValueGenerator($default, $reflectionParameter));
+                $type = $parameter->getType();
+
+                if ($default->getValue() === null && strpos($type ?? '?', '?') !== 0 && strpos($type, '|') === false) {
+                    $parameter->setType('?' . $type);
+                }
             }
 
             $method->setParameter($parameter);
diff --git a/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodPrefixInterceptor.php b/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodPrefixInterceptor.php
index 4b2cbe7e..5fa22a1a 100644
--- a/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodPrefixInterceptor.php
+++ b/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodPrefixInterceptor.php
@@ -27,7 +27,7 @@ public function __construct(PropertyGenerator $prefixInterceptor)
 
         $interceptor = new ParameterGenerator('prefixInterceptor');
 
-        $interceptor->setType(Closure::class);
+        $interceptor->setType('?' . Closure::class);
         $interceptor->setDefaultValue(null);
         $this->setParameter(new ParameterGenerator('methodName', 'string'));
         $this->setParameter($interceptor);
diff --git a/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodSuffixInterceptor.php b/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodSuffixInterceptor.php
index 90a1ac05..e9ba9f16 100644
--- a/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodSuffixInterceptor.php
+++ b/src/ProxyManager/ProxyGenerator/AccessInterceptor/MethodGenerator/SetMethodSuffixInterceptor.php
@@ -27,7 +27,7 @@ public function __construct(PropertyGenerator $suffixInterceptor)
 
         $interceptor = new ParameterGenerator('suffixInterceptor');
 
-        $interceptor->setType(Closure::class);
+        $interceptor->setType('?' . Closure::class);
         $interceptor->setDefaultValue(null);
         $this->setParameter(new ParameterGenerator('methodName', 'string'));
         $this->setParameter($interceptor);
diff --git a/src/ProxyManager/ProxyGenerator/LazyLoadingGhost/MethodGenerator/SetProxyInitializer.php b/src/ProxyManager/ProxyGenerator/LazyLoadingGhost/MethodGenerator/SetProxyInitializer.php
index 54ed2562..0f9e3111 100644
--- a/src/ProxyManager/ProxyGenerator/LazyLoadingGhost/MethodGenerator/SetProxyInitializer.php
+++ b/src/ProxyManager/ProxyGenerator/LazyLoadingGhost/MethodGenerator/SetProxyInitializer.php
@@ -22,7 +22,7 @@ public function __construct(PropertyGenerator $initializerProperty)
     {
         parent::__construct(
             'setProxyInitializer',
-            [(new ParameterGenerator('initializer', Closure::class))->setDefaultValue(null)],
+            [(new ParameterGenerator('initializer', '?' . Closure::class))->setDefaultValue(null)],
             self::FLAG_PUBLIC,
             '$this->' . $initializerProperty->getName() . ' = $initializer;'
         );
diff --git a/src/ProxyManager/ProxyGenerator/LazyLoadingValueHolder/MethodGenerator/SetProxyInitializer.php b/src/ProxyManager/ProxyGenerator/LazyLoadingValueHolder/MethodGenerator/SetProxyInitializer.php
index b2998c79..9c896e7b 100644
--- a/src/ProxyManager/ProxyGenerator/LazyLoadingValueHolder/MethodGenerator/SetProxyInitializer.php
+++ b/src/ProxyManager/ProxyGenerator/LazyLoadingValueHolder/MethodGenerator/SetProxyInitializer.php
@@ -27,7 +27,7 @@ public function __construct(PropertyGenerator $initializerProperty)
 
         $initializerParameter = new ParameterGenerator('initializer');
 
-        $initializerParameter->setType(Closure::class);
+        $initializerParameter->setType('?' . Closure::class);
         $initializerParameter->setDefaultValue(null);
         $this->setParameter($initializerParameter);
         $this->setBody('$this->' . $initializerProperty->getName() . ' = $initializer;');
diff --git a/tests/ProxyManagerTestAsset/AccessInterceptorValueHolderMock.php b/tests/ProxyManagerTestAsset/AccessInterceptorValueHolderMock.php
index 38cc472d..372ef702 100644
--- a/tests/ProxyManagerTestAsset/AccessInterceptorValueHolderMock.php
+++ b/tests/ProxyManagerTestAsset/AccessInterceptorValueHolderMock.php
@@ -45,12 +45,12 @@ public static function staticProxyConstructor($instance, $prefixInterceptors, $s
         return $selfInstance;
     }
 
-    public function setMethodPrefixInterceptor(string $methodName, \Closure $prefixInterceptor = null) : void
+    public function setMethodPrefixInterceptor(string $methodName, ?\Closure $prefixInterceptor = null) : void
     {
         // no-op (on purpose)
     }
 
-    public function setMethodSuffixInterceptor(string $methodName, \Closure $suffixInterceptor = null) : void
+    public function setMethodSuffixInterceptor(string $methodName, ?\Closure $suffixInterceptor = null) : void
     {
         // no-op (on purpose)
     }
diff --git a/tests/ProxyManagerTestAsset/LazyLoadingMock.php b/tests/ProxyManagerTestAsset/LazyLoadingMock.php
index 6a936236..5426a9e6 100644
--- a/tests/ProxyManagerTestAsset/LazyLoadingMock.php
+++ b/tests/ProxyManagerTestAsset/LazyLoadingMock.php
@@ -35,7 +35,7 @@ public static function staticProxyConstructor($initializer) : self
         return $instance;
     }
 
-    public function setProxyInitializer(\Closure $initializer = null) : void
+    public function setProxyInitializer(?\Closure $initializer = null) : void
     {
         $this->initializer = $initializer;
     }