diff --git a/gtk2_ardour/mixer_ui.cc b/gtk2_ardour/mixer_ui.cc
index 40735a0635..299c9fc4f3 100644
--- a/gtk2_ardour/mixer_ui.cc
+++ b/gtk2_ardour/mixer_ui.cc
@@ -24,6 +24,7 @@
 #include <gtkmm/accelmap.h>
 
 #include "pbd/convert.h"
+#include "pbd/stacktrace.h"
 #include <glibmm/thread.h>
 
 #include <gtkmm2ext/gtk_ui.h>
@@ -403,6 +404,10 @@ Mixer_UI::sync_order_keys (string const & src)
 			changed = true;
 		}
 	}
+
+	if (keys.size() != rows.size()) {
+		PBD::stacktrace (cerr, 20);
+	}
 	assert(keys.size() == rows.size());
 
 	// Remove any gaps in keys caused by automation children tracks