diff --git a/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php b/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php
index 615a820..840e4bf 100644
--- a/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php
+++ b/core/lib/Drupal/Core/Routing/RouteSubscriberBase.php
@@ -68,7 +68,7 @@ public function onDynamicRoutes(RouteBuildEvent $event) {
    */
   public function onAlterRoutes(RouteBuildEvent $event) {
     $collection = $event->getRouteCollection();
-    $this->alterRoutes($collection, $event->getModule());
+    return $this->alterRoutes($collection, $event->getModule());
   }
 
 }