diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 7c0b223..8226541 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -21,6 +21,7 @@
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Render\Markup;
+use Drupal\Component\Utility\Xss;
 
 /**
  * @defgroup content_flags Content markers