Affiliates + Membership plugin: sales registration is wrong

I'm installing the affiliates plugin. I've set up a membership site using the membership plugin. I have set up 3 membership levels: free trial, 1 month mms and 1 year mms.

When I create a new account through a generated affiliate link, the number of unique clicks, signups and referred paid members is updated correcty, but no credits are added. When I check the affiliate system administration, it apears that all referred members are registered for the free plan (I tried signing up for a free mms first and upgrading later, as well as upgrading instantly, without creating a free account first).

What could be causing this?