Crystal reports - division by zero

I am having problems with a division by zero error - my formula is

    (100 / (SUM({rep_v_JobAnalysis.ActSales}, {@GroupBy})) - (100 / SUM({rep_v_JobAnalysis.ActAgyComm}, {@GroupBy}))

    *

    SUM({@ActProfit}, {@GroupBy}))

It throws the error on

100 / (SUM({rep_v_JobAnalysis.ActSales}))

I have added several different ways of adding formula for if = 0 then 0 but it still keeps throwing back the error

Any ideas guys would be much appreciated - thanks