As usual, Bob raises a good point. What I usually do, if I have a field that will be used as a divisor, such as
Field1/Field2
is use
Nz(Field2, 1)
so I'd end up with
Nz(Field1, 0) / Nz(Field2, 1)
If Field2 is left Null, then Field1 will be divided by 1, which actually means that it's left undivided. If the Null Field2 was simply overlooked, no error will be raised. On the other hand, if the user left Field2 blank, on purpose, because they didn't want Field1 divided, that will also be accomplished.