diff --git a/src/dispextern.h b/src/dispextern.h
index 4d98f3f1dfd..50b2d4368bb 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -2795,6 +2795,7 @@ int merge_faces P_ ((struct frame *, Lisp_Object, int, int));
 int compute_char_face P_ ((struct frame *, int, Lisp_Object));
 void free_all_realized_faces P_ ((Lisp_Object));
 extern Lisp_Object Qforeground_color, Qbackground_color;
+extern Lisp_Object Qframe_set_background_mode;
 extern char unspecified_fg[], unspecified_bg[];
 void free_realized_multibyte_face P_ ((struct frame *, int));