It is already in the render() because its inherited from the [TwigEngine in Bridge](https://github.com/Tobion/symfony/blob/282eb0430d665efafbba07f67bbf6835e31d1923/src/Symfony/Bridge/Twig/TwigEngine.php#L46). It just needs to be added here for renderResponse() which is new for Symfony\Bundle\FrameworkBundle\Templating\EngineInterface.