Getting User ID in Email Template is not Working

I think there is a bug in amember Email Template Hook. I am using this code which should pull the user ID to whom confirmation/new product purchase Welcome email is sent but its not working.

Am_Di::getInstance()->hook->add(Am_Event::MAIL_TEMPLATE_BEFORE_PARSE, 'sample_template_parse');
function sample_template_parse(Am_Event $event){
$user = $event->getUserId(); //This is NOT returning the ID
}