Free Trial subscription automatically added to paid subscription member

Hello

I'm using Membership Premium 2.1.4, Buddypress 1.5.4 and WordPress 3.3.1

I have two Membership subscription levels: free trial and paid. When a user signs up for the paid "premium" subscription, they are assigned to it, but they are also assigned to the regular free trial subscription.

Is that supposed to happen? I can't see where it's getting set in the admin screens. They are two separate subscription plans, with two separate membership levels as starting points. I didn't expect them to have a sequential relationship. How can I configure the plugin so that the free and the trial subscriptions are two separate entities with separate levels? I'd like users to have either one or the other, not both.

thanks,

Clare