diff --git a/core/modules/book/lib/Drupal/book/Controller/BookController.php b/core/modules/book/lib/Drupal/book/Controller/BookController.php
index ecede91..9e27cd4 100644
--- a/core/modules/book/lib/Drupal/book/Controller/BookController.php
+++ b/core/modules/book/lib/Drupal/book/Controller/BookController.php
@@ -143,7 +143,8 @@ public function bookExport($type, NodeInterface $node) {
       throw new NotFoundHttpException();
     }
 
-    return new Response(drupal_render($this->bookExport->{$method}($node)));
+    $rendered_html = drupal_render($this->bookExport->{$method}($node));
+    return new Response($rendered_html);
   }
 
 }