diff --git a/src/transparency_gui.cpp b/src/transparency_gui.cpp
--- a/src/transparency_gui.cpp
+++ b/src/transparency_gui.cpp
@@ -6,6 +6,7 @@
 #include "table/strings.h"
 #include "functions.h"
 #include "gui.h"
+#include "window_gui.h"
 #include "viewport.h"
 #include "gfx.h"
 #include "sound.h"