First off, do yourself a favour and never use division in a Form or Report without trapping for zero. I use IIF because it makes the most structural sense to me. You go...
SUM([Field1] / IIF([Field2]>0, [Field2],1))
As far as the overflow goes I know only that you have somehow specified a calculation that is too complicated. Is it possible that either of the fields that you use are caluculated themselves? Are both fields proper data types?
Hope this helps
Chris