Fatal error: Cannot use object of type WP_Error as array in /home/mypath/wp-content/plugins/membersh

When trying to signup using WordPress 3.2.1 and Buddypress 1.5.1 with extended profile fields in the registration form and using the simple invite codes option, I get the following error when trying to sign-up and NOT entering one of the assigned simple invite codes.

Fatal error: Cannot use object of type WP_Error as array in /home/mypath/wp-content/plugins/membership/membershipincludes/addons/simple.invitecodes.php on line 89

If I sign-up with the simple invite code entered, it works perfectly. But without, instead of displaying an error, I get the above message.

Thanks

Pete