JSONSerialization的响应。swift 3中方法POST的jsonObject为零

这是一个例子,我想做的事情,但线 if let json = try JSONSerialization.jsonObject(with: data) as? [String: String] false 因为 JSONSerialization.jsonObject 是 nil

func parser(lbl: UILabel){
    let postString = "xxx=xxx&xxxx=xxxx==&xxxxx=xxxxx&xxxxxx=xxxxxx&xx=xx"

    let url = URL(string: "http://xxxxxxxxxx.com/xxxxxx/xxxxx/xxxxxx.php")!
    var request = URLRequest(url: url)

    request.httpMethod = "POST"
    request.httpBody = postString.data(using: .utf8)

    let task = URLSession.shared.dataTask(with: request) { data, response, error in
        guard let data = data else {
            lbl.text = "error";

            return
        }

        do {
            if let json = try JSONSerialization.jsonObject(with: data) as? [String: String] {
                DispatchQueue.main.async {

                    let error = Int(json["success"]!)

                    let message = json["message"]
                    lbl.text = message
                }
            } 
        } catch let parseError {
            print("error to parse: \(parseError)")

            let responseString = String(data: data, encoding: .utf8)
            print("response`enter code here` : \(responseString!)")
        }
    }
    task.resume()
}