diff --git a/src/widgets/main_widget.h b/src/widgets/main_widget.h
--- a/src/widgets/main_widget.h
+++ b/src/widgets/main_widget.h
@@ -11,7 +11,7 @@
 #define WIDGETS_MAIN_WIDGET_H
 
 /** Widgets of the #MainWindow class. */
-enum MainWidgets {
+enum MainWidgets : WidgetID {
 	WID_M_VIEWPORT, ///< Main window viewport.
 };