diff --git a/core/lib/Drupal/Core/Routing/Router.php b/core/lib/Drupal/Core/Routing/Router.php
index 6fb0b23c07..580effbe7f 100644
--- a/core/lib/Drupal/Core/Routing/Router.php
+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -194,6 +194,8 @@ public function matchRequest(Request $request) {
       if ($this->checkAccess) {
         $this->checkAccess($request);
       }
+      // We can not return $parameters because the access check can change the
+      // request attributes.
       return $request->attributes->all();
     }
 
@@ -417,7 +419,7 @@ protected function sortFilters() {
   }
 
   /**
-   * Apply access check service to the route and parameters in the request.
+   * Applies access check service to the route and parameters in the request.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The request to access check.