How to get columns that doesn't contain any data in SSRS Report

I have an ssrs Report which contains amounts for the months....but for some months there is no amountif there is no amount that month is hiding in my reporti have tried the below expression but iam not getting the months that contains no amounts and i want to place 0 in place of amounts for the months that contains no amounts.....=iif(SUM(Fields!Performance_Amount_Usd.Value),SUM(Fields!Performance_Amount_Usd.Value),0)can any one suggest me how to get all the months and how to place zero for the months that contains no amount.........