php - Mysqli->insert_id not returning anything

I'm creating a login/sign up page with PHP and MySQL, everything works just fine, but when I try to retrieve the insert_id from MySQL just doesn't work. Even if I try to "print" the result, it only says 1. Heres my code:

Here's my connection:

<?
class Connection { //Connect to database
    /* Arguments */
    private $_host = "localhost";
    private $_username = ""; /* Change */
    private $_password = ""; /* Change */
    private $_database = "";
    private $_connection;
    private static $_instance;

    /* Methods */
    private function __construct(){ //Initial method
        $this->_connection=new mysqli($this->_host,$this->_username,$this->_password,$this->_database);
        if($this->_connection->connect_error){
           trigger_error("Error al conectar a base de datos: " . $this->_connection->connect_error);
        }
        $this->_connection->set_charset("utf8");
    }
    private function __clone(){} //Magic method
    public static function getInstance(){ //Get instance of connection
        if(!self::$_instance){
            self::$_instance=new self();
        }
        return self::$_instance;
    }
    public function getConnection(){ //Get MySQLi connection
        return $this->_connection;
    }
}

class Query extends Connection { //Query database
    public static function run($sql){
        return parent::getInstance()->getConnection()->query($sql);
    }
}
?>
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And here's my users class:

<?
class Users {
    /* Arguments */
    public static $result=NULL;

    /* Methods */
    public static function getDuplicated($sql){ //Get duplicated records
        $d=Query::run($sql);
        self::$result=$d->num_rows;
    }

    public static function createUser($sql){ //Create new user on database
        $c=Query::run($sql);
        self::$result=$c->insert_id;
    }
}
?>
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And finally this is where I call the INSERT:

<?
Users::$result=NULL;
$c=Users::createUser("INSERT INTO users (user,password,lastLogin) VALUES ('$username_register','$password_register',NOW())");
echo Users::$result;
?>
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As I said, everything works just fine, I can get records from database, it inserts correctly and so on, but when I user insert-id, it just gives nothing.