>Steve,
>
>I tried your suggestion and it really works!
>My situation, however would require a great amount of work to solve using this method as I don't subclass the basic controls. My doubt is: Is this behaviour by default? I can't remember having seeing this situation in the other applications I worked with! I feel I am missing something...
>

I can't duplicate your problem with a value of 0 stored in the control source variable. Perhaps your control source doesn't contain zero after all?