diff --git a/src/ProxyManager/Generator/MethodGenerator.php b/src/ProxyManager/Generator/MethodGenerator.php
index 1349c4e7..ae45ec64 100644
--- a/src/ProxyManager/Generator/MethodGenerator.php
+++ b/src/ProxyManager/Generator/MethodGenerator.php
@@ -69,7 +69,7 @@ public static function copyMethodSignature(MethodReflection $reflectionMethod):
                 $parameter->setDefaultValue(new ValueGenerator($default, $reflectionParameter));
                 $type = $parameter->getType();
 
-                if ($default->getValue() === null && strpos($type ?? '?', '?') !== 0 && strpos($type, '|') === false) {
+                if ($default->getValue() === null && strpos($type ?? '?', '?') !== 0 && strpos($type, '|') === false && $type !== 'mixed') {
                     $parameter->setType('?' . $type);
                 }
             }
diff --git a/tests/ProxyManagerTest/Generator/MethodGeneratorTest.php b/tests/ProxyManagerTest/Generator/MethodGeneratorTest.php
index d06ae613..9a8d53ee 100644
--- a/tests/ProxyManagerTest/Generator/MethodGeneratorTest.php
+++ b/tests/ProxyManagerTest/Generator/MethodGeneratorTest.php
@@ -10,6 +10,7 @@
 use ProxyManager\Generator\MethodGenerator;
 use ProxyManagerTestAsset\BaseClass;
 use ProxyManagerTestAsset\ClassWithAbstractPublicMethod;
+use ProxyManagerTestAsset\ClassWithNullDefaultMethodArguments;
 use ProxyManagerTestAsset\EmptyClass;
 use ProxyManagerTestAsset\ReturnTypeHintedClass;
 use ProxyManagerTestAsset\ScalarTypeHintedClass;
@@ -134,6 +135,27 @@ public function scalarTypeHintedMethods(): array
         ];
     }
 
+    /**
+     * @requires PHP 8.0
+     */
+    public function testGenerateMethodWithNullDefaultMixedArgument(): void
+    {
+        $method = MethodGenerator::fromReflectionWithoutBodyAndDocBlock(new MethodReflection(
+            ClassWithNullDefaultMethodArguments::class,
+            'acceptMixed'
+        ));
+
+        self::assertSame('acceptMixed', $method->getName());
+
+        $parameters = $method->getParameters();
+
+        self::assertCount(1, $parameters);
+
+        $param = $parameters['param'];
+
+        self::assertSame('mixed', $param->getType());
+    }
+
     public function testGenerateMethodWithVoidReturnTypeHinting(): void
     {
         $method = MethodGenerator::fromReflectionWithoutBodyAndDocBlock(new MethodReflection(
diff --git a/tests/ProxyManagerTestAsset/ClassWithNullDefaultMethodArguments.php b/tests/ProxyManagerTestAsset/ClassWithNullDefaultMethodArguments.php
new file mode 100644
index 00000000..be6f298f
--- /dev/null
+++ b/tests/ProxyManagerTestAsset/ClassWithNullDefaultMethodArguments.php
@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace ProxyManagerTestAsset;
+
+class ClassWithNullDefaultMethodArguments
+{
+    public function acceptMixed(mixed $param = null)
+    {
+        return $param;
+    }
+}