Looking for some help.

Getting a Division by 0 error in my report.
My code below is working out the target v actual percentage for sales.

Sum ({@ItemsNet}, {@month})/{@Target} *100 - 100

Problem I got is that we are doing this by product group so some months there are no sales for a product group which I think is the reason for my error, just don't know how I can write into the code to ignore these instances. Works absolutely fine for the product groups with sales every month.

Any help appreciated.