Arithmetic over/underflow error on form open

I've tried finding a solution to this on my own using other examples from previous forum posts, but nothing has worked. I have a form containing some fields that perform a simple "present value" calculation. The calculation works, but when you open the form, I receive the following error:

Of course, all of the fields involved in the calculation are blank when the form opens, so there's nothing to calculate, which is, I think, what is causing this error.

Is there some formcalc script I can use to prevent this error?

Thanks for any help you can offer!
Jamie