diff --git a/demos/ctk-demo/overlay2.c b/demos/ctk-demo/overlay2.c
index 3cf47ee1c7..0d65f0d91d 100644
--- a/demos/ctk-demo/overlay2.c
+++ b/demos/ctk-demo/overlay2.c
@@ -20,7 +20,7 @@ margin_changed (CtkAdjustment *adjustment,
 }
 
 CtkWidget *
-do_overlay2 (CtkWidget *do_widget)
+do_overlay2 (CtkWidget *do_widget G_GNUC_UNUSED)
 {
   static CtkWidget *window = NULL;