diff --git a/libs/ardour/panner_shell.cc b/libs/ardour/panner_shell.cc
index 0297cba6ef..75543433c5 100644
--- a/libs/ardour/panner_shell.cc
+++ b/libs/ardour/panner_shell.cc
@@ -405,6 +405,7 @@ PannerShell::set_bypassed (bool yn)
 	}
 	
 	_bypassed = yn;
+	_session.set_dirty ();
 	Changed (); /* EMIT SIGNAL */
 }