Ruerte

Try the solution from Bloodrayne1995 ->

http://ww.wp.xz.cn/support/topic/fatal-error-1139?replies=3

WORK 4 ME!

Code:

require_once(ABSPATH . WPINC . '/ms-functions.php');
$id = get_user_by('email',$id_or_email);
if(is_bool($id)){
$username = "Notch";
}else{
$username = $id->user_login;
}