diff --git a/src/Controller/ViewerController.php b/src/Controller/ViewerController.php
index e1059f83..bf35410f 100644
--- a/src/Controller/ViewerController.php
+++ b/src/Controller/ViewerController.php
@@ -10,6 +10,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+declare(strict_types=1);
+
 namespace Drupal\collabora_online\Controller;
 
 use Drupal\collabora_online\Cool\CollaboraDiscovery;
@@ -96,7 +98,7 @@ public function editor(MediaInterface $media, Request $request, $edit = FALSE) {
     $render_array['#attached']['library'][] = 'collabora_online/cool.frame';
 
     $response = new Response();
-    $response->setContent($this->renderer->renderRoot($render_array));
+    $response->setContent((string) $this->renderer->renderRoot($render_array));
 
     return $response;
   }
diff --git a/src/Cool/CollaboraDiscovery.php b/src/Cool/CollaboraDiscovery.php
index 3c3b285b..2f761e4d 100644
--- a/src/Cool/CollaboraDiscovery.php
+++ b/src/Cool/CollaboraDiscovery.php
@@ -10,6 +10,8 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
+declare(strict_types=1);
+
 namespace Drupal\collabora_online\Cool;
 
 use Drupal\collabora_online\Exception\CollaboraNotAvailableException;