WARNING message from Memberships Plugin

I created a couple different levels of membership. Then, I updated one user to reflect him as a “visitor” level of membership and as soon as I did, this message below popped up on my screen, and I’d like to know what it means and how to fix it as necessary.

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

Warning: Cannot modify header information – headers already sent by (output started at /home/shawnmon/public_html/wp-includes/wp-db.php:990) in /home/shawnmon/public_html/wp-includes/pluggable.php on line 876