public function add($username, $repository, $issue, $labels)
    {
        if (is_string($labels)) {
            $labels = [$labels];
        } elseif (0 === count($labels)) {
            throw new InvalidArgumentException('The labels parameter should be a single label or an array of labels');
        }

        return $this->post('/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/issues/'.rawurlencode($issue).'/labels', $labels);
    }