diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index 8214a5d..271c8a4 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -777,7 +777,10 @@ function _menu_translate(&$router_item, $map, $to_arg = FALSE) {
   $router_item['tab_parent_href'] = implode('/', $tab_parent_map);
   $router_item['options'] = array();
   if (!empty($router_item['route_name'])) {
+    // Route-provided menu items do not have menu loaders, so replace the map
+    // with the link map.
     $map = $link_map;
+
     $route_provider = Drupal::getContainer()->get('router.route_provider');
     $route = $route_provider->getRouteByName($router_item['route_name']);
     $router_item['access'] = menu_item_route_access($route, $router_item['href'], $map);
@@ -967,8 +970,7 @@ function menu_item_route_access(Route $route, $href, &$map) {
   // Attempt to match this path to provide a fully built request to the
   // access checker.
   try {
-    $attributes = Drupal::service('router.dynamic')->matchRequest($request);
-    $request->attributes->add($attributes);
+    $request->attributes->add(Drupal::service('router.dynamic')->matchRequest($request));
   }
   catch (NotFoundHttpException $e) {
     return FALSE;
@@ -978,9 +980,13 @@ function menu_item_route_access(Route $route, $href, &$map) {
   $path_bits = explode('/', trim($route->getPath(), '/'));
   foreach ($map as $index => $map_item) {
     $matches = array();
+    // Search for placeholders wrapped by curly braces. For example, a path
+    // 'foo/{bar}/baz' would return 'bar'.
     if (isset($path_bits[$index]) && preg_match('/{(?<placeholder>.*)}/', $path_bits[$index], $matches)) {
-      if (isset($attributes[$matches['placeholder']])) {
-        $map[$index] = $attributes[$matches['placeholder']];
+      // If that placeholder is present on the request attributes, replace the
+      // placeholder in the map with the value.
+      if ($request->attributes->has($matches['placeholder'])) {
+        $map[$index] = $request->attributes->get($matches['placeholder']);
       }
     }
   }