public function changeAvatar($nick, $id_avatar)
    {
        if (!is_numeric($id_avatar) || 0 >= $id_avatar) {
            throw new InvalidArgumentException(
                "RealtimeChangeAvatar id_avatar field should be positive integer", 404);
        }

        if (empty($nick)) {
            throw new InvalidArgumentException(
                "RealtimeChangeAvatar nick field should not be empty", 404);
        }

        $command = $this->client->getCommand('RealtimeChangeAvatar', array('nick' => $nick, 'id' => $id_avatar));
        return $this->executeCommand($command);
    }