a.rb

diff --git a/core/lib/Drupal/Core/Routing/RouteBuilder.php b/core/lib/Drupal/Core/Routing/RouteBuilder.php
index 76a826b..dd894a4 100644
--- a/core/lib/Drupal/Core/Routing/RouteBuilder.php
+++ b/core/lib/Drupal/Core/Routing/RouteBuilder.php
@@ -119,8 +119,13 @@ public function rebuild() {
         foreach ($routes['route_callbacks'] as $route_callback) {
           $callback = $this->controllerResolver->getControllerFromDefinition($route_callback);
           if ($callback_routes = call_user_func($callback)) {
-            foreach ($callback_routes as $name => $callback_route) {
-              $collection->add($name, $callback_route);
+            if (is_array($callback_routes)) {
+              foreach ($callback_routes as $name => $callback_route) {
+                $collection->add($name, $callback_route);
+              }
+            }
+            else {
+              $collection->addCollection($callback_routes);
             }
           }
         }