MYSQLi num_rows always returns 0

MYSQLi num_rows always returns 0

I am trying to use MySQLi with my website and I am writting a login
script. Now i have a problem with num_rows always returning value 0
So this is my code
if($query = $mysqli->prepare("SELECT 'password','ban' FROM users WHERE
'username'=? LIMIT 1")){
$query->bind_param('s', $username);
$query->execute();
$query->bind_result($db_password,$ban);
$query->store_result();
$query->fetch();
$numrows = $query->num_rows;
if($numrows == 1){
if($ban == 0)
{
if($db_password == $password)
{
$_SESSION['login'] = 'ok';
//$_SESSION['id'] = $id;
//$_SESSION['realname'] =
preg_replace("/[^a-zA-Z0-9_\-]+/","",$realname);
header("Location: /ematura/index.php");
$query->close();
}else{
die("Napaka");
}
}else{
die("Raèun zaklenjen");
}
}
My question is, why is num_rows always 0, even though the user exists?