diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 139fb0f..e8f0a04 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -2557,7 +2557,9 @@ function system_preprocess_block(&$variables) {
 
       // System menu blocks should get the same class as menu module blocks.
       default:
-        if (in_array($variables['block']->delta, array_keys(menu_list_system_menus()))) {
+        // Strip off 'menu-' prefix from block delta.
+        $delta = substr($variables['block']->delta, 5);
+        if (in_array($delta, array_keys(menu_list_system_menus()))) {
           $variables['attributes']['role'] = 'navigation';
           $variables['attributes']['class'][] = 'block-menu';
         }