diff --git a/gtk2_ardour/pianokeyboard.cc b/gtk2_ardour/pianokeyboard.cc
index 090e1392c2..ae508fdd7a 100644
--- a/gtk2_ardour/pianokeyboard.cc
+++ b/gtk2_ardour/pianokeyboard.cc
@@ -917,6 +917,8 @@ APianoKeyboard::recompute_dimensions ()
 		}
 	}
 
+	assert (number_of_white_keys > 0);
+
 	int width  = get_width ();
 	int height = get_height ();