warning on subscribers page in admin

I’ve a warning appearing on my subscribers page in the admin

Warning: Missing argument 2 for wpdb::prepare(), called in /homepages/46/d77376898/htdocs/wylie/wp-content/plugins/newsletter-pro/users.php on line 242 and defined in /homepages/46/d77376898/htdocs/wylie/wp-includes/wp-db.php on line 1147

any ideas?