Report >> Expression #Num appear need to

I have a report with a calculated field, the problem arises when I preview or print the report and one of the (2 in total) fields which it uses in the calculation is zero. This displaying the #Num! error. I need code to hide this expression (text box or label, not sure) if one of the two fields are equal to zero. I know that using the properties I can build code in the On Activate event. Please help.