Error with subscription if coupon is used and amount set to 0
I am using coupons with ultimate membership pro, I can set the discount from 1% to 99% and have no issues with the redirection to PayPal, however when I put the coupon to a discount of 100% PayPal gives me the "Invalid Regular period. You must specify valid values for the A3, P3, and T3 parameters for a subscription." message. How can I change it to work?