public function addPostField($field, $value) {
        if (is_null($this->postData)) {
            $this->postData = array();
        }

        if (is_array($this->postData)) {
            $this->postData[$field] = (string) $value;
            return $this;
        } else {
            throw new LogicException('cannot set field for non array');
        }
    }