diff --git a/demos/gtk-demo/hsla.h b/demos/gtk-demo/hsla.h
index 52c6bdc146..9ce8003113 100644
--- a/demos/gtk-demo/hsla.h
+++ b/demos/gtk-demo/hsla.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <gdk/gdk.h>
+
 typedef struct _GdkHSLA GdkHSLA;
 
 struct _GdkHSLA {