카카오페이 asp.net 400 오류뜹니다

[nahakmin]

using System;
using System.Net;
using System.IO;
using System.Collections.Generic;
using System.Text;
using System.Windows;
using Newtonsoft.Json.Linq;

public partial class test3 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
string url = “https://kapi.kakao.com/v1/payment/ready”;

    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
    req.Method = "POST";
    req.Host = "kapi.kakao.com";
    req.Headers.Add("Authorization", "KakaoAK xxxx50f51dc38d946ebdb79f3461xxxx");
    req.ContentType = "application/x-www-form-urlencoded;charset=utf-8";

    var json = new JObject();
    json.Add("cid", "TC0ONETIME");
    json.Add("partner_order_id", "111");
    json.Add("partner_user_id", "namuseung");
    json.Add("item_name", "Kimmuseung");
    json.Add("quantity", 1);
    json.Add("total_amount", 1000);
    json.Add("tax_free_amount", 10);
    json.Add("approval_url", "https://developers.kakao.com/success");
    json.Add("cancel_url", "https://developers.kakao.com/cancel");
    json.Add("fail_url", "https://developers.kakao.com/fail");

    //Response.Write(json.ToString());
    //Response.End();


    // data들을 UTF8형식으로 바이트 변환함.            
    //byte[] bytes = Encoding.UTF8.GetBytes(builder.ToString());
    byte[] bytes = Encoding.UTF8.GetBytes(json.ToString());

    req.ContentLength = bytes.Length;

    // HttpWebRequest req에 변환된 데이터들을 넣자.
    using (Stream reqStream = req.GetRequestStream())
        reqStream.Write(bytes, 0, bytes.Length);

    // 받은 내용 저장 변수
    string responseText = string.Empty;

    // HttpWebRequest req로부터 response한 결과 클래스
    using (WebResponse resp = req.GetResponse())
    {
        Stream respStream = resp.GetResponseStream();
    }
}

}