diff --git a/GenericRuntime.php b/GenericRuntime.php
index c88832f..fa93648 100644
--- a/GenericRuntime.php
+++ b/GenericRuntime.php
@@ -86,7 +86,7 @@ public function __construct(array $options = [])
     /**
      * {@inheritdoc}
      */
-    public function getResolver(callable $callable, \ReflectionFunction $reflector = null): ResolverInterface
+    public function getResolver(callable $callable, ?\ReflectionFunction $reflector = null): ResolverInterface
     {
         if (!$callable instanceof \Closure) {
             $callable = \Closure::fromCallable($callable);
diff --git a/Runner/Symfony/ConsoleApplicationRunner.php b/Runner/Symfony/ConsoleApplicationRunner.php
index 430ce57..ee8a762 100644
--- a/Runner/Symfony/ConsoleApplicationRunner.php
+++ b/Runner/Symfony/ConsoleApplicationRunner.php
@@ -27,7 +27,7 @@ class ConsoleApplicationRunner implements RunnerInterface
     private $input;
     private $output;
 
-    public function __construct(Application $application, ?string $defaultEnv, InputInterface $input, OutputInterface $output = null)
+    public function __construct(Application $application, ?string $defaultEnv, InputInterface $input, ?OutputInterface $output = null)
     {
         $this->application = $application;
         $this->defaultEnv = $defaultEnv;
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.