[Plugin: Register Plus] Fatal Error, Need Help

I am getting this error when someone tries to register:

Fatal error: Call to undefined function: prepare() in wp-content/plugins/register-plus/register-plus.php on line 1223

I made a manual edit so my line 1223 is:

$plaintext_pass = $wpdb->prepare($_POST[‘user_pw’]);

any help would be appreciated. i am running WP 2.2.1