diff --git a/core/includes/menu.inc b/core/includes/menu.inc
index d56c53b..1508004 100644
--- a/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -2401,7 +2401,7 @@ function menu_link_get_preferred($path = NULL, $selected_menu = NULL) {
 
   if (empty($selected_menu)) {
     // Use an illegal menu name as the key for the preferred menu link.
-    $selected_menu = '__preferred__';
+    $selected_menu = MENU_PREFERRED_LINK;
   }
 
   if (!isset($preferred_links[$path])) {
@@ -2459,9 +2459,9 @@ function menu_link_get_preferred($path = NULL, $selected_menu = NULL) {
             _menu_translate($candidate_item, $map);
             if ($candidate_item['access']) {
               $preferred_links[$path][$menu_name] = $candidate_item;
-              if (empty($preferred_links[$path]['__preferred__'])) {
+              if (empty($preferred_links[$path][MENU_PREFERRED_LINK])) {
                 // Store the most specific link.
-                $preferred_links[$path]['__preferred__'] = $candidate_item;
+                $preferred_links[$path][MENU_PREFERRED_LINK] = $candidate_item;
               }
             }
           }
diff --git a/core/modules/menu/menu.module b/core/modules/menu/menu.module
index b10a17a..5a735cc 100644
--- a/core/modules/menu/menu.module
+++ b/core/modules/menu/menu.module
@@ -18,6 +18,20 @@
 const MENU_MAX_MENU_NAME_LENGTH_UI = 27;
 
 /**
+ * Reserved key to identify the most specific menu link for a given path.
+ *
+ * The value of this constant is a hash of the constant name. We use the hash
+ * so that the reserved key is over 32 characters in length and will not
+ * collide with allowed menu names:
+ * @code
+ * sha1('MENU_PREFERRED_LINK') = 1cf698d64d1aa4b83907cf6ed55db3a7f8e92c91
+ * @endcode
+ *
+ * @see menu_link_get_preferred()
+ */
+const MENU_PREFERRED_LINK = '1cf698d64d1aa4b83907cf6ed55db3a7f8e92c91';
+
+/**
  * Implements hook_help().
  */
 function menu_help($path, $arg) {