ftp_rawlist returns false when connected

ftp_rawlist return false and I'm not able to say why

This is my code

/**
 * Wrapper for ftp_rawlist
 *
 * @param string $dir
 * @param bool $recursive
 * @return array an array where each element corresponds to one line of text.
 */
public function rawlist($dir = "/", $recursive = false)
{
    if (!$this->_conn) {
        throw new \Exception(__CLASS__ . " - no connection established with server");
    }
    $dir = $this->correctFilePath($dir);
    return @ftp_rawlist($this->_conn, $dir, $recursive);
}

dir value is the default one and I don't trigger the no connection exception so I suppose that I'm properly connected.

I'm connecting this way

/**
 * Connect to server using connect string
 * Connection string: ftp://user:pass@server:port/path
 * user,pass,port,path are optional parts
 *
 * @param string $string
 * @param int $timeout
 * @return void
 * @throws \Exception
 */
public function connect($string, $timeout = 900)
{
    $params = $this->validateConnectionString($string);
    $port = isset($params['port']) ? (int)$params['port'] : 21;

    $this->_conn = ftp_connect($params['host'], $port, $timeout);

    if (!$this->_conn) {
        throw new \Exception("Cannot connect to host: {$params['host']}");
    }
    if (isset($params['user']) && isset($params['pass'])) {
        $this->login($params['user'], $params['pass']);
    } else {
        $this->login();
    }
    if (isset($params['path'])) {
        if (!$this->chdir($params['path'])) {
            throw new \Exception("Cannot chdir after login to: {$params['path']}");
        }
    }
}

What am I missing ?