How to remove NaN% value and replace with 0% when there is no value

I have this expression in my SSRS report. I use Business Intelligence Development Studio 2008

I have coded value values of those field and they coming from code. Calculation works fine but I am getting NaN% instead of 0% when there is no value.I need to display result as 0% when there is no value.

= IIF(String.IsNullOrEmpty(Fields!7DaysMADischarge.Value),"", CStr(Math.Round((Fields!7DaysMADischarge.Value / Fields!MADischarge.Value)*100)) +"%")

Any help will be very much appreciated. Thanks in advance.