Membership giving error on new user registration/scubscription

Hi

When new user registers and signs up to my free plan, he gets the following error:

Notice: Undefined offset: 4 in Sites/AMCF/wp-content/plugins/membership/membershipincludes/classes/membershippublic.php on line 2253

Warning: Cannot modify header information - headers already sent by (output started at Sites/AMCF/wp-content/plugins/membership/membershipincludes/classes/membershippublic.php:2253) in Sites/AMCF/wp-includes/pluggable.php on line 876

I'm not quite sure why?