diff --git a/src/widgets/dropdown_func.h b/src/widgets/dropdown_func.h
--- a/src/widgets/dropdown_func.h
+++ b/src/widgets/dropdown_func.h
@@ -12,6 +12,8 @@
 #ifndef WIDGETS_DROPDOWN_FUNC_H
 #define WIDGETS_DROPDOWN_FUNC_H
 
+#include "../window_gui.h"
+
 /* Show drop down menu containing a fixed list of strings */
 void ShowDropDownMenu(Window *w, const StringID *strings, int selected, int button, uint32 disabled_mask, uint32 hidden_mask, uint width = 0);