diff --git a/src/Drupal/DrupalExtension/ServiceContainer/DrupalExtension.php b/src/Drupal/DrupalExtension/ServiceContainer/DrupalExtension.php
index a72a406f..8c1c26f8 100644
--- a/src/Drupal/DrupalExtension/ServiceContainer/DrupalExtension.php
+++ b/src/Drupal/DrupalExtension/ServiceContainer/DrupalExtension.php
@@ -39,7 +39,7 @@ class DrupalExtension implements ExtensionInterface
    *
    * @param null|ServiceProcessor $processor
    */
-    public function __construct(ServiceProcessor $processor = null)
+    public function __construct(?ServiceProcessor $processor = null)
     {
         $this->processor = $processor ? : new ServiceProcessor();
     }