using a System Property in FxCop build runner shows error

I was trying to us a templated build configuration with a FxCop runner. I tried defining parameterised limits for errors and warnings, but if I use the %parameter% rather than a number in "Errors limit:" field I get this error "Errors limit must be a positive number or zero" when I try to save. Is this a bug or am I doing something wrong?

I am on TC 6.5.2.