diff --git a/gtk2_ardour/luainstance.h b/gtk2_ardour/luainstance.h
index 43e5bb86b3..4b146e7ba0 100644
--- a/gtk2_ardour/luainstance.h
+++ b/gtk2_ardour/luainstance.h
@@ -40,6 +40,10 @@ namespace luabridge {
 	class LuaRef;
 }
 
+namespace Gtk {
+	class Window;
+}
+
 typedef std::bitset<LuaSignal::LAST_SIGNAL> ActionHook;
 
 class LuaCallback : public ARDOUR::SessionHandlePtr, public sigc::trackable