diff --git a/Packages/Qt_widget/demo/Qt_widget/Min_quadrilateral_2/Qt_widget_toolbar_layers.h b/Packages/Qt_widget/demo/Qt_widget/Min_quadrilateral_2/Qt_widget_toolbar_layers.h
index 3ae0cf1e798..15dd163b123 100644
--- a/Packages/Qt_widget/demo/Qt_widget/Min_quadrilateral_2/Qt_widget_toolbar_layers.h
+++ b/Packages/Qt_widget/demo/Qt_widget/Min_quadrilateral_2/Qt_widget_toolbar_layers.h
@@ -25,7 +25,7 @@
 #include <CGAL/Cartesian.h>
 #include <CGAL/Polygon_2.h>
 #include <CGAL/IO/Qt_widget.h>
-
+#include <qmainwindow.h>
 #include "min_quadrilateral_layers.h"
 
 #include <qobject.h>