Report Builder "division by zero"

I have a question about Report Builder. (actually may be an easy question
for some? just not me.)
I have created a report including job costs. My problem is that in one of
the calculations, there is division. If we have a job that does not have
cost or a completed qty, there is division that includes a 0, therefore I
get the "?????????" error in calculation. When I do a total on a field that
has "???????" I receive the same answer "?????????????". I know in the
aggregate field I can click on Ignore Unknown or 0, but this does not work.
Is there a way to have the aggregate ignore an error?

Thank you in advance for your help,