diff --git a/gtk2_ardour/route_ui.cc b/gtk2_ardour/route_ui.cc
index e1b9da4b89..7d9c10e29f 100644
--- a/gtk2_ardour/route_ui.cc
+++ b/gtk2_ardour/route_ui.cc
@@ -2009,6 +2009,13 @@ RouteUI::parameter_changed (string const & p)
 void
 RouteUI::setup_invert_buttons ()
 {
+	uint32_t const N = _route ? _route->phase_control()->size() : 0;
+
+	if (_n_polarity_invert == N) {
+		return;
+	}
+	_n_polarity_invert = N;
+
 	/* remove old invert buttons */
 	for (vector<ArdourButton*>::iterator i = _invert_buttons.begin(); i != _invert_buttons.end(); ++i) {
 		_invert_button_box.remove (**i);
@@ -2016,11 +2023,10 @@ RouteUI::setup_invert_buttons ()
 
 	_invert_buttons.clear ();
 
-	if (!_route) {
+	if (N == 0) {
 		return;
 	}
 
-	uint32_t const N = _route->phase_control()->size();
 	uint32_t const to_add = (N <= _max_invert_buttons) ? N : 1;
 
 	for (uint32_t i = 0; i < to_add; ++i) {
diff --git a/gtk2_ardour/route_ui.h b/gtk2_ardour/route_ui.h
index 59bae48549..92f161e3dd 100644
--- a/gtk2_ardour/route_ui.h
+++ b/gtk2_ardour/route_ui.h
@@ -345,6 +345,7 @@ private:
 	int _i_am_the_modifier;
 	std::vector<ArdourWidgets::ArdourButton*> _invert_buttons;
 	Gtk::Menu* _invert_menu;
+	uint32_t   _n_polarity_invert;
 
 	StripableColorDialog _color_picker;