diff --git a/gtk2_ardour/gui_thread.h b/gtk2_ardour/gui_thread.h
index 0e88ad5d77..a62a098654 100644
--- a/gtk2_ardour/gui_thread.h
+++ b/gtk2_ardour/gui_thread.h
@@ -1,6 +1,8 @@
 #ifndef __ardour_gtk_gui_thread_h__
 #define __ardour_gtk_gui_thread_h__
 
+#include <gtkmm2ext/gtk_ui.h>
+
 #define ENSURE_GUI_THREAD(slot) \
      if (!Gtkmm2ext::UI::instance()->caller_is_gui_thread()) {\
 	Gtkmm2ext::UI::instance()->call_slot ((slot));\