public function getMergeFields()
    {
        // since fev2017, MailChimp API doesn't handle null value en throw 400
        // errors when you try to add subscriber with a mergefields value to null
        foreach ($this->mergeFields as $key => $value) {
            if ($value === null) {
                unset($this->mergeFields[$key]);
            }
        }
        return $this->mergeFields;
    }