diff -u b/core/lib/Drupal/Core/Config/FileStorage.php b/core/lib/Drupal/Core/Config/FileStorage.php
--- b/core/lib/Drupal/Core/Config/FileStorage.php
+++ b/core/lib/Drupal/Core/Config/FileStorage.php
@@ -163,9 +163,9 @@
   public static function getNamesWithPrefix($prefix = '') {
     // @todo Use $this->getPath() to allow for contextual search of files in
     //   custom paths.
-    $files = glob(config_get_config_directory() . '/' . $prefix . '*.' . self::getFileExtension());
+    $files = glob(config_get_config_directory() . '/' . $prefix . '*.' . FileStorage::getFileExtension());
     $clean_name = function ($value) {
-      return basename($value, '.' . self::getFileExtension());
+      return basename($value, '.' . FileStorage::getFileExtension());
     };
     return array_map($clean_name, $files);
   }