Register Plus and Autochimp

The Autochimp plugin
http://www.wandererllc.com/company/plugins/autochimp/
doesn’t not work properly with Register Plus
http://wordpress.org/extend/plugins/register-plus/

When a new user registers in the blog using Register Plus he introduces their firstname and lastname but then the Autochimp plugin can’t send this information to the configured mailchimp list.

The code in the Autochimp plugin seems fine

$merge_vars = array('FNAME'=>$user_info->first_name, 'LNAME'=>$user_info->last_name);

                                        switch( $mode )
                                        {
                                                case MMU_ADD:
                                                {
                                                        // By default this sends a confirmation email - you will not see new members
                                                        // until the link contained in it is clicked!
                                                        $retval = $api->listSubscribe( $list_id, $user_info->user_email, $merge_vars );
                                                }

Could it be a sync problem?

Thanks