GtkEntry "insert-at-cursor" not working

I am trying to write an image resizer and I need to get two boxes to update when one of them is updated (the width and height boxes, so that fixed-aspect is possible)  I figured out that I should use the insert-at-cursor of a GtkEntry to make that feasible.  I have the following code:

$txtWidth = new GtkEntry();
$txtWidth->connect_simple("insert-at-cursor","test");

Theoretically, it should work, but test() never gets called.  If I change "insert-at-cursor" to "backspace", it works (test() is called).

Any ideas? Am I going about this the wrong way?

-HalfBrian