Another option (especially if you've got a report with many expressions that could result in divide by zero situations is to use a Custom Code function.

In the Code tab/window of Report Properties, enter something like the following:

Public Function DivideBy(ByVal Exp1, ByVal Exp2)

If Exp2 = 0 Then

DivideBy = 0

Else : DivideBy = Exp1 / Exp2

End If

End Function

Then insert the expression

=code.DivideBy(Field!ToBeDivided.Value,Field!DividingBy.Value)

into any cell that has the potential for divide by zero problems.