database error in Membership plugin

I get this error often (only in the admin), depending on what I save:

Warning: Missing argument 2 for wpdb::prepare(), called in /home/book/public_html/plus/wp-content/plugins/membership/membershipincludes/classes/class.membership.php on line 447 and defined in /home/book/public_html/plus/wp-includes/wp-db.php on line 990