present value - Why is PV(tax shield) calculated using cost of debt capital for discounting?

I understand that (interest payment)×(corporation tax) is the cash flow saving (assumed to go on in perpetuity) and it can be written as (debt)×(cost of debt capital)×(corporation tax). But why is it discounted with cost of debt capital? If it is a cash outflow saving on the whole operation of the company, why isn't is discounted with WACC?