diff -u b/core/modules/system/system.module b/core/modules/system/system.module
--- b/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -901,7 +901,7 @@
     \Drupal::service('module_listing')->reset();
     $modules_cache = \Drupal::service('module_listing')->listExtensions();
     $files = array();
-    ksort(modules_cache);
+    ksort($modules_cache);
     foreach ($modules_cache as $name => $module) {
       $files[$name] = $module->getPathname();
     }