diff --git a/fixtures/f013/A.php b/fixtures/f013/A.php
index aa0393d..ac70988 100644
--- a/fixtures/f013/A.php
+++ b/fixtures/f013/A.php
@@ -2,6 +2,7 @@
 
 namespace DeepCopy\f013;
 
+use BadMethodCallException;
 use Doctrine\Persistence\Proxy;
 
 class A implements Proxy
@@ -11,14 +12,15 @@ class A implements Proxy
     /**
      * @inheritdoc
      */
-    public function __load()
+    public function __load(): void
     {
     }
 
     /**
      * @inheritdoc
      */
-    public function __isInitialized()
+    public function __isInitialized(): bool
     {
+        throw new BadMethodCallException();
     }
 }
diff --git a/fixtures/f013/B.php b/fixtures/f013/B.php
index 89425d8..d1382e6 100644
--- a/fixtures/f013/B.php
+++ b/fixtures/f013/B.php
@@ -2,6 +2,7 @@
 
 namespace DeepCopy\f013;
 
+use BadMethodCallException;
 use Doctrine\Persistence\Proxy;
 
 class B implements Proxy
@@ -11,15 +12,16 @@ class B implements Proxy
     /**
      * @inheritdoc
      */
-    public function __load()
+    public function __load(): void
     {
     }
 
     /**
      * @inheritdoc
      */
-    public function __isInitialized()
+    public function __isInitialized(): bool
     {
+        throw new BadMethodCallException();
     }
 
     public function getFoo()
diff --git a/src/DeepCopy/DeepCopy.php b/src/DeepCopy/DeepCopy.php
index 3937542..4555d42 100644
--- a/src/DeepCopy/DeepCopy.php
+++ b/src/DeepCopy/DeepCopy.php
@@ -89,9 +89,11 @@ public function skipUncloneable($skipUncloneable = true)
     /**
      * Deep copies the given object.
      *
-     * @param mixed $object
+     * @template TObject
      *
-     * @return mixed
+     * @param TObject $object
+     *
+     * @return TObject
      */
     public function copy($object)
     {
@@ -270,7 +272,9 @@ function ($object) {
             }
         }
 
-        $property->setAccessible(true);
+        if (PHP_VERSION_ID < 80100) {
+            $property->setAccessible(true);
+        }
 
         // Ignore uninitialized properties (for PHP >7.4)
         if (method_exists($property, 'isInitialized') && !$property->isInitialized($object)) {
diff --git a/src/DeepCopy/Filter/Doctrine/DoctrineCollectionFilter.php b/src/DeepCopy/Filter/Doctrine/DoctrineCollectionFilter.php
index e6d9377..66e91e5 100644
--- a/src/DeepCopy/Filter/Doctrine/DoctrineCollectionFilter.php
+++ b/src/DeepCopy/Filter/Doctrine/DoctrineCollectionFilter.php
@@ -19,7 +19,9 @@ public function apply($object, $property, $objectCopier)
     {
         $reflectionProperty = ReflectionHelper::getProperty($object, $property);
 
-        $reflectionProperty->setAccessible(true);
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }
         $oldCollection = $reflectionProperty->getValue($object);
 
         $newCollection = $oldCollection->map(
diff --git a/src/DeepCopy/Filter/Doctrine/DoctrineEmptyCollectionFilter.php b/src/DeepCopy/Filter/Doctrine/DoctrineEmptyCollectionFilter.php
index 7b33fd5..fa1c034 100644
--- a/src/DeepCopy/Filter/Doctrine/DoctrineEmptyCollectionFilter.php
+++ b/src/DeepCopy/Filter/Doctrine/DoctrineEmptyCollectionFilter.php
@@ -21,7 +21,9 @@ class DoctrineEmptyCollectionFilter implements Filter
     public function apply($object, $property, $objectCopier)
     {
         $reflectionProperty = ReflectionHelper::getProperty($object, $property);
-        $reflectionProperty->setAccessible(true);
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }
 
         $reflectionProperty->setValue($object, new ArrayCollection());
     }
diff --git a/src/DeepCopy/Filter/ReplaceFilter.php b/src/DeepCopy/Filter/ReplaceFilter.php
index 7aca593..fda8e72 100644
--- a/src/DeepCopy/Filter/ReplaceFilter.php
+++ b/src/DeepCopy/Filter/ReplaceFilter.php
@@ -30,7 +30,9 @@ public function __construct(callable $callable)
     public function apply($object, $property, $objectCopier)
     {
         $reflectionProperty = ReflectionHelper::getProperty($object, $property);
-        $reflectionProperty->setAccessible(true);
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }
 
         $value = call_user_func($this->callback, $reflectionProperty->getValue($object));
 
diff --git a/src/DeepCopy/Filter/SetNullFilter.php b/src/DeepCopy/Filter/SetNullFilter.php
index bea86b8..6722272 100644
--- a/src/DeepCopy/Filter/SetNullFilter.php
+++ b/src/DeepCopy/Filter/SetNullFilter.php
@@ -18,7 +18,9 @@ public function apply($object, $property, $objectCopier)
     {
         $reflectionProperty = ReflectionHelper::getProperty($object, $property);
 
-        $reflectionProperty->setAccessible(true);
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }
         $reflectionProperty->setValue($object, null);
     }
 }
diff --git a/src/DeepCopy/Matcher/PropertyTypeMatcher.php b/src/DeepCopy/Matcher/PropertyTypeMatcher.php
index c7f4690..7980bfa 100644
--- a/src/DeepCopy/Matcher/PropertyTypeMatcher.php
+++ b/src/DeepCopy/Matcher/PropertyTypeMatcher.php
@@ -39,7 +39,9 @@ public function matches($object, $property)
             return false;
         }
 
-        $reflectionProperty->setAccessible(true);
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }
 
         // Uninitialized properties (for PHP >7.4)
         if (method_exists($reflectionProperty, 'isInitialized') && !$reflectionProperty->isInitialized($object)) {
diff --git a/tests/DeepCopyTest/Matcher/Doctrine/DoctrineProxyMatcherTest.php b/tests/DeepCopyTest/Matcher/Doctrine/DoctrineProxyMatcherTest.php
index 22e380d..38de911 100644
--- a/tests/DeepCopyTest/Matcher/Doctrine/DoctrineProxyMatcherTest.php
+++ b/tests/DeepCopyTest/Matcher/Doctrine/DoctrineProxyMatcherTest.php
@@ -39,7 +39,7 @@ class FooProxy implements Proxy
     /**
      * @inheritdoc
      */
-    public function __load()
+    public function __load(): void
     {
         throw new BadMethodCallException();
     }
@@ -47,7 +47,7 @@ public function __load()
     /**
      * @inheritdoc
      */
-    public function __isInitialized()
+    public function __isInitialized(): bool
     {
         throw new BadMethodCallException();
     }