diff --git a/core/lib/Drupal/Core/Routing/Router.php b/core/lib/Drupal/Core/Routing/Router.php
index cb1505761b..373c368764 100644
--- a/core/lib/Drupal/Core/Routing/Router.php
+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -105,7 +105,7 @@ public function addRouteFilter(RouteFilterInterface $route_filter) {
    *   The route enhancer.
    */
   public function addRouteEnhancer(RouteEnhancerInterface $route_enhancer) {
-    $this->enhancers = $route_enhancer;
+    $this->enhancers[] = $route_enhancer;
   }
 
   /**
@@ -253,9 +253,8 @@ protected function getInitialRouteCollection(Request $request) {
    *   from route enhancers.
    */
   protected function applyRouteEnhancers($defaults, Request $request) {
-    /** @var \Drupal\Core\Routing\Enhancer\RouteEnhancerInterface[] $enhancers */
     foreach ($this->enhancers as $enhancer) {
-      if ($enhancer->applies($defaults[RouteObjectInterface::ROUTE_OBJECT])) {
+      if (!$enhancer instanceof RouteEnhancerInterface || $enhancer->applies($defaults[RouteObjectInterface::ROUTE_OBJECT])) {
         $defaults = $enhancer->enhance($defaults, $request);
       }
     }