public function getUri()
    {
        if (empty($this->getConfig('email')) === true) {
            throw new \RuntimeException('No email specified.');
        }
        $uri = 'http://www.gravatar.com/avatar/' . md5(strtolower(trim($this->getConfig('email')))) . '.jpg';
        $args = http_build_query([
            's' => $this->getConfig('size'),
            'd' => $this->getConfig('defaultUri'),
        ]);
        if (empty($args) === false) {
            $uri .= '?' . $args;
        }
        return $uri;
    }