Adding products to Price Groups

How do you do this?

I have some VATable (20%) items - Hardware and

Some Zero rate VAT - food.

I have created two price groups - Zero VAT and 20%VAT.

If, when pricing I leave the 20%VAT empty and put a value in Zero VAT, I get the following error:

Error loading Razor Script uCommerce/Product.cshtml
Unit price cannot be null when adding product to basket.

SO,

How do I mix and match VAT and Non VAT products.