protected content coupon not allowing a one off discount

Hi. I am having a small issue with the coupons in protected content.

It appears that any discount applied with the coupon is applied for the full term of a membership.

I would like to be able to offer a discount on the first period only and not forever as it seem to at the moment.

Am I missing something or is there a way to restrict the discount to the first period of membership only.

Thanks

Stef.