diff -u b/core/lib/Drupal/Core/Template/Loader/FilesystemLoader.php b/core/lib/Drupal/Core/Template/Loader/FilesystemLoader.php
--- b/core/lib/Drupal/Core/Template/Loader/FilesystemLoader.php
+++ b/core/lib/Drupal/Core/Template/Loader/FilesystemLoader.php
@@ -52,10 +52,13 @@
    *
-   * @param string $path A path where to look for templates
-   * @param string $namespace A path name
+   * @param string $path
+   *   A path where to look for templates.
+   * @param string $namespace
+   *   (optional) A path name.
    */
   public function addPath($path, $namespace = self::MAIN_NAMESPACE) {
     // Invalidate the cache.
     $this->cache = array();
     $this->paths[$namespace][] = rtrim($path, '/\\');
   }
+
 }