Discounts with negative quantity value

If you create an invoice, and then add a line item of "10% discount", when you put in a value for the VAT which is discounted, and therefore a negative sum, you get an error stating that all VAT levels need to be positive.

For example, item A is £100 VAT, item B is £200 VAT, so total is £300 VAT. 10% discount is applied, so item C is -0.1 x £300 VAT, which is £-30 and £-6.00 VAT. This VAT value is rejected.