Invalid dimension name in Design Table

I am relatively new to SW 2012 (many years experience with ProE) and have an issue.

I created a design table and have changed the dimension name from D1@SKETCH1 to SS@SKETCH1 and I keep getting an error message stating "Column heading ss@sketch1 contains and invalid dimension name "SS"

Are there chracters that I can and cannot use? In Pro-E I never had this issue

Thanks

John

SolidworksConfigurations design Tables