diff --git a/gtk2_ardour/transport_masters_dialog.cc b/gtk2_ardour/transport_masters_dialog.cc
index 0b2d1c4a81..1a2c058ea3 100644
--- a/gtk2_ardour/transport_masters_dialog.cc
+++ b/gtk2_ardour/transport_masters_dialog.cc
@@ -273,6 +273,7 @@ TransportMastersWidget::rebuild ()
 	}
 
 	update_usability ();
+	allow_master_select (_session && !_session->config.get_external_sync());
 }
 
 bool
@@ -682,7 +683,6 @@ TransportMastersWidget::set_session (ARDOUR::Session* s)
 
 	if (_session) {
 		_session->config.ParameterChanged.connect (session_config_connection, invalidator (*this), boost::bind (&TransportMastersWidget::param_changed, this, _1), gui_context());
-		allow_master_select (!_session->config.get_external_sync());
 		rebuild ();
 	} else {
 		clear ();