public function customersUpload(array $customers, $site = null)
    {
        if (! count($customers)) {
            throw new \InvalidArgumentException(
                'Parameter `customers` must contains array of the customers'
            );
        }

        /* @noinspection PhpUndefinedMethodInspection */
        return $this->client->makeRequest(
            '/customers/upload',
            "POST",
            $this->fillSite($site, ['customers' => json_encode($customers)])
        );
    }