Membership - auto-assign custom user roles

Hi,

I'm using WPMU membership on http://www.yogatravel.co.uk.

Everything works fine, except that the associated user role is not assigned upon successful subscription sign-up.

I am using custom user roles ("advertiser-five") but all the members are assigned "subscriber" role.

It is selected correctly in the Access Levels section.

Any ideas?

Cheers,

David