diff --git a/src/widgets/timetable_widget.h b/src/widgets/timetable_widget.h
--- a/src/widgets/timetable_widget.h
+++ b/src/widgets/timetable_widget.h
@@ -11,7 +11,7 @@
 #define WIDGETS_TIMETABLE_WIDGET_H
 
 /** Widgets of the #TimetableWindow class. */
-enum VehicleTimetableWidgets {
+enum VehicleTimetableWidgets : WidgetID {
 	WID_VT_CAPTION,                     ///< Caption of the window.
 	WID_VT_ORDER_VIEW,                  ///< Order view.
 	WID_VT_TIMETABLE_PANEL,             ///< Timetable panel.