Hosting with free renewal domain seems not working

Hello, i have a product with the option "Offer a free domain registration/transfer and free renewal (if product is renewed)"
When whmcs generate the renew invoice it displays two item: the product's renew and the domain's renew, but the domain's renew is not set to 0 but to the regular price.
How can I fix them if i want to give free domain renew?
Thanks