diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 9dc24c9..a9a46d5 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -187,7 +187,6 @@ function book_menu() {
   );
   $items['book/export/%/%node'] = array(
     'route_name' => 'book_export',
-    'type' => MENU_CALLBACK,
   );
   $items['node/%node/outline'] = array(
     'title' => 'Outline',
diff --git a/core/modules/book/lib/Drupal/book/Controller/BookController.php b/core/modules/book/lib/Drupal/book/Controller/BookController.php
index 33dcdd7..9e06fc2 100644
--- a/core/modules/book/lib/Drupal/book/Controller/BookController.php
+++ b/core/modules/book/lib/Drupal/book/Controller/BookController.php
@@ -6,6 +6,7 @@
 
 namespace Drupal\book\Controller;
 
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\ControllerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException;
@@ -112,7 +113,7 @@ public function bookRender() {
    * @throws \Symfony\Component\HttpKernel\Exception\NotFoundHttpException
    */
   public function bookExport($type, EntityInterface $node) {
-    $type = drupal_strtolower($type);
+    $type = Unicode::strtolower($type);
 
     $export_function = 'book_export_' . $type;