diff --git a/lisp/outline.el b/lisp/outline.el
index bfba50e061b..012396c2eb0 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -37,6 +37,9 @@
 
 ;;; Code:
 
+(defvar font-lock-warning-face)
+
+
 (defgroup outlines nil
   "Support for hierarchical outlining."
   :prefix "outline-"