I have some formulas in my reports, and to prevent divsion by zero I do like this in the expression field:
=IIF(Fields!F1.Value <> 0, Fields!F2.Value/Fields!F1.Va
However, you can use
if Fields!F1.Value <> 0 then Fields!F2.Value/Fields!F1.Value else 0
which should work, since it doesn't evaluate the then clause if the "if" section is false.