protected function callDriver($sMethod, $aArguments = [], $sDriver = null)
    {
        //  Work out which driver we need to use
        if (empty($sDriver)) {
            $oDriver = $this->oEnabledDriver;
        } elseif (array_key_exists($sDriver, $this->aDrivers)) {
            $oDriver = $this->aDrivers[$sDriver];
        } else {
            throw new DriverException('"' . $sDriver . '" is not a valid CDN driver.');
        }

        $oStorageDriver = Factory::service('StorageDriver', 'nails/module-cdn');
        $oInstance      = $oStorageDriver->getInstance($oDriver->slug);

        if (empty($oInstance)) {
            throw new DriverException('Failed to load CDN driver instance.');
        }

        return call_user_func_array([$oInstance, $sMethod], $aArguments);
    }