diff --git a/gtk2_ardour/mixer_strip.h b/gtk2_ardour/mixer_strip.h
index 9bd811a2f7..d1ab3c398e 100644
--- a/gtk2_ardour/mixer_strip.h
+++ b/gtk2_ardour/mixer_strip.h
@@ -259,6 +259,8 @@ class MixerStrip : public RouteUI, public Gtk::EventBox
 	void revert_to_default_display ();
 
 	static int scrollbar_height;
+	
+	void update_io_button (boost::shared_ptr<ARDOUR::Route> route, Width width, bool input_button);
 };
 
 #endif /* __ardour_mixer_strip__ */