freight rates table allows non numeric characters to be saved

Hi,

Im Marketpress, if you set the table rates for freight, you can save a non numeric in the rate. like $8 for instance.

see my screenshot here: http://monosnap.com/image/rzbCNUBNLCtTwBRxu33H0dwJr

I think it should not allow you to save with a $ in the field.