diff --git a/GenericRuntime.php b/GenericRuntime.php
index f1b3254..645c7b1 100644
--- a/GenericRuntime.php
+++ b/GenericRuntime.php
@@ -83,7 +83,7 @@ public function __construct(array $options = [])
         $this->options = $options;
     }
 
-    public function getResolver(callable $callable, \ReflectionFunction $reflector = null): ResolverInterface
+    public function getResolver(callable $callable, ?\ReflectionFunction $reflector = null): ResolverInterface
     {
         $callable = $callable(...);
         $parameters = ($reflector ?? new \ReflectionFunction($callable))->getParameters();
diff --git a/RuntimeInterface.php b/RuntimeInterface.php
index 757468c..f151757 100644
--- a/RuntimeInterface.php
+++ b/RuntimeInterface.php
@@ -23,7 +23,7 @@ interface RuntimeInterface
      *
      * The callable itself should return an object that represents the application to pass to the getRunner() method.
      */
-    public function getResolver(callable $callable, \ReflectionFunction $reflector = null): ResolverInterface;
+    public function getResolver(callable $callable, ?\ReflectionFunction $reflector = null): ResolverInterface;
 
     /**
      * Returns a callable that knows how to run the passed object and that returns its exit status as int.