Need help keeping focus on a field

I have a form with a calculated field on it.  A-B/A, for example.

If A is 0, then I want to tell the user that A cannot be equal to 0 and the cursor should go back to A until they enter a different value.

At this point, I get a system error about dividing by 0 even before I get the message I give the user.

I would appreciate any help on how to accomplish this.