diff --git a/composer.json b/composer.json
index 8660b2a0..5d509621 100644
--- a/composer.json
+++ b/composer.json
@@ -18,8 +18,8 @@
             "homepage": "http://ocramius.github.io/"
         },
         {
-            "name": "Nicolas Grekas",
-            "email": "p@tchwork.com"
+            "name":     "Nicolas Grekas",
+            "email":    "p@tchwork.com"
         }
     ],
     "replace": {
@@ -36,7 +36,7 @@
     },
     "require-dev": {
         "ext-phar":                     "*",
-        "symfony/phpunit-bridge":       "^5.2"
+        "symfony/phpunit-bridge":       "^5.2|^6.0"
     },
     "autoload": {
         "psr-4": {
diff --git a/src/ProxyManager/Generator/MagicMethodGenerator.php b/src/ProxyManager/Generator/MagicMethodGenerator.php
index ac4ba269..ea7447b1 100644
--- a/src/ProxyManager/Generator/MagicMethodGenerator.php
+++ b/src/ProxyManager/Generator/MagicMethodGenerator.php
@@ -4,6 +4,7 @@
 
 namespace ProxyManager\Generator;
 
+use Laminas\Code\Generator\ParameterGenerator;
 use ReflectionClass;
 
 use function strtolower;
@@ -14,7 +15,7 @@
 class MagicMethodGenerator extends MethodGenerator
 {
     /**
-     * @param mixed[] $parameters
+     * @param ParameterGenerator[]|array[]|string[] $parameters
      */
     public function __construct(ReflectionClass $originalClass, string $name, array $parameters = [])
     {
diff --git a/src/ProxyManager/GeneratorStrategy/FileWriterGeneratorStrategy.php b/src/ProxyManager/GeneratorStrategy/FileWriterGeneratorStrategy.php
index 7f8bd551..36cdb481 100644
--- a/src/ProxyManager/GeneratorStrategy/FileWriterGeneratorStrategy.php
+++ b/src/ProxyManager/GeneratorStrategy/FileWriterGeneratorStrategy.php
@@ -44,7 +44,7 @@ public function __construct(FileLocatorInterface $fileLocator)
     public function generate(ClassGenerator $classGenerator): string
     {
         $generatedCode = $classGenerator->generate();
-        $className     = $classGenerator->getNamespaceName() . '\\' . $classGenerator->getName();
+        $className     = (string) $classGenerator->getNamespaceName() . '\\' . $classGenerator->getName();
         $fileName      = $this->fileLocator->getProxyFileName($className);
 
         set_error_handler($this->emptyErrorHandler);
diff --git a/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/InitializationTrackerTest.php b/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/InitializationTrackerTest.php
index 31b5aa8d..008e0b81 100644
--- a/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/InitializationTrackerTest.php
+++ b/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/InitializationTrackerTest.php
@@ -23,8 +23,10 @@ protected function createProperty(): PropertyGenerator
 
     public function testInitializationFlagIsFalseByDefault(): void
     {
-        $property = $this->createProperty();
+        $defaultValue = $this->createProperty()
+            ->getDefaultValue();
 
-        self::assertFalse($property->getDefaultValue()->getValue());
+        self::assertNotNull($defaultValue);
+        self::assertFalse($defaultValue->getValue());
     }
 }
diff --git a/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/PrivatePropertiesMapTest.php b/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/PrivatePropertiesMapTest.php
index 3eb76d99..598dc626 100644
--- a/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/PrivatePropertiesMapTest.php
+++ b/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/PrivatePropertiesMapTest.php
@@ -30,27 +30,29 @@ protected function createProperty(): PropertyGenerator
 
     public function testExtractsProtectedProperties(): void
     {
-        $map = new PrivatePropertiesMap(
+        $defaultValue = (new PrivatePropertiesMap(
             Properties::fromReflectionClass(new ReflectionClass(ClassWithMixedProperties::class))
-        );
+        ))->getDefaultValue();
 
+        self::assertNotNull($defaultValue);
         self::assertSame(
             [
                 'privateProperty0' => [ClassWithMixedProperties::class => true],
                 'privateProperty1' => [ClassWithMixedProperties::class => true],
                 'privateProperty2' => [ClassWithMixedProperties::class => true],
             ],
-            $map->getDefaultValue()->getValue()
+            $defaultValue->getValue()
         );
     }
 
     public function testSkipsAbstractProtectedMethods(): void
     {
-        $map = new PrivatePropertiesMap(
+        $defaultValue = (new PrivatePropertiesMap(
             Properties::fromReflectionClass(new ReflectionClass(ClassWithAbstractProtectedMethod::class))
-        );
+        ))->getDefaultValue();
 
-        self::assertSame([], $map->getDefaultValue()->getValue());
+        self::assertNotNull($defaultValue);
+        self::assertSame([], $defaultValue->getValue());
     }
 
     public function testIsStaticPrivate(): void
diff --git a/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/ProtectedPropertiesMapTest.php b/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/ProtectedPropertiesMapTest.php
index d225d577..2f999400 100644
--- a/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/ProtectedPropertiesMapTest.php
+++ b/tests/ProxyManagerTest/ProxyGenerator/LazyLoadingGhost/PropertyGenerator/ProtectedPropertiesMapTest.php
@@ -29,27 +29,29 @@ protected function createProperty(): PropertyGenerator
 
     public function testExtractsProtectedProperties(): void
     {
-        $map = new ProtectedPropertiesMap(
+        $defaultValue = (new ProtectedPropertiesMap(
             Properties::fromReflectionClass(new ReflectionClass(ClassWithMixedProperties::class))
-        );
+        ))->getDefaultValue();
 
+        self::assertNotNull($defaultValue);
         self::assertSame(
             [
                 'protectedProperty0' => ClassWithMixedProperties::class,
                 'protectedProperty1' => ClassWithMixedProperties::class,
                 'protectedProperty2' => ClassWithMixedProperties::class,
             ],
-            $map->getDefaultValue()->getValue()
+            $defaultValue->getValue()
         );
     }
 
     public function testSkipsAbstractProtectedMethods(): void
     {
-        $map = new ProtectedPropertiesMap(
+        $defaultValue = (new ProtectedPropertiesMap(
             Properties::fromReflectionClass(new ReflectionClass(ClassWithAbstractProtectedMethod::class))
-        );
+        ))->getDefaultValue();
 
-        self::assertSame([], $map->getDefaultValue()->getValue());
+        self::assertNotNull($defaultValue);
+        self::assertSame([], $defaultValue->getValue());
     }
 
     public function testIsStaticPrivate(): void
diff --git a/tests/ProxyManagerTest/ProxyGenerator/PropertyGenerator/PublicPropertiesMapTest.php b/tests/ProxyManagerTest/ProxyGenerator/PropertyGenerator/PublicPropertiesMapTest.php
index ade49f03..1b2bb84b 100644
--- a/tests/ProxyManagerTest/ProxyGenerator/PropertyGenerator/PublicPropertiesMapTest.php
+++ b/tests/ProxyManagerTest/ProxyGenerator/PropertyGenerator/PublicPropertiesMapTest.php
@@ -26,8 +26,11 @@ public function testEmptyClass(): void
             Properties::fromReflectionClass(new ReflectionClass(EmptyClass::class))
         );
 
-        self::assertIsArray($publicProperties->getDefaultValue()->getValue());
-        self::assertEmpty($publicProperties->getDefaultValue()->getValue());
+        $defaultValue = $publicProperties->getDefaultValue();
+
+        self::assertNotNull($defaultValue);
+        self::assertIsArray($defaultValue->getValue());
+        self::assertEmpty($defaultValue->getValue());
         self::assertTrue($publicProperties->isStatic());
         self::assertSame('private', $publicProperties->getVisibility());
         self::assertTrue($publicProperties->isEmpty());
@@ -39,9 +42,12 @@ public function testClassWithPublicProperties(): void
             Properties::fromReflectionClass(new ReflectionClass(ClassWithPublicProperties::class))
         );
 
+        $defaultValue = $publicProperties->getDefaultValue();
+
         self::assertTrue($publicProperties->isStatic());
         self::assertSame('private', $publicProperties->getVisibility());
         self::assertFalse($publicProperties->isEmpty());
+        self::assertNotNull($defaultValue);
         self::assertSame(
             [
                 'property0' => true,
@@ -55,21 +61,24 @@ public function testClassWithPublicProperties(): void
                 'property8' => true,
                 'property9' => true,
             ],
-            $publicProperties->getDefaultValue()->getValue()
+            $defaultValue->getValue()
         );
     }
 
     public function testClassWithMixedProperties(): void
     {
+        $defaultValue = (new PublicPropertiesMap(
+            Properties::fromReflectionClass(new ReflectionClass(ClassWithMixedProperties::class))
+        ))->getDefaultValue();
+
+        self::assertNotNull($defaultValue);
         self::assertSame(
             [
                 'publicProperty0' => true,
                 'publicProperty1' => true,
                 'publicProperty2' => true,
             ],
-            (new PublicPropertiesMap(
-                Properties::fromReflectionClass(new ReflectionClass(ClassWithMixedProperties::class))
-            ))->getDefaultValue()->getValue()
+            $defaultValue->getValue()
         );
     }
 }
diff --git a/tests/ProxyManagerTest/Signature/ClassSignatureGeneratorTest.php b/tests/ProxyManagerTest/Signature/ClassSignatureGeneratorTest.php
index e191d921..b6c32ca8 100644
--- a/tests/ProxyManagerTest/Signature/ClassSignatureGeneratorTest.php
+++ b/tests/ProxyManagerTest/Signature/ClassSignatureGeneratorTest.php
@@ -37,10 +37,13 @@ public function testAddSignature(): void
             ->expects(self::once())
             ->method('addPropertyFromGenerator')
             ->with(self::callback(static function (PropertyGenerator $property): bool {
+                $defaultValue = $property->getDefaultValue();
+
                 return $property->getName() === 'signaturePropertyName'
                     && $property->isStatic()
                     && $property->getVisibility() === 'private'
-                    && $property->getDefaultValue()->getValue() === 'valid-signature';
+                    && $defaultValue !== null
+                    && $defaultValue->getValue() === 'valid-signature';
             }));
 
         $this