Twitter身份验证错误

Question

我正在尝试获取Twitter REST api的授权令牌，但似乎我做错了什么。我的代码有问题吗？

//Authorization
var customerKey = "xxxxxxxxxx";
var customerSecret = "xxxxxxxxxxxxxxxxxxx";
var b64 = Convert.ToBase64String(Encoding.UTF8.GetBytes(string.Format("{0}:{1}", WebUtility.UrlEncode(customerKey), WebUtility.UrlEncode(customerSecret))));
var req = new HttpRequestMessage(HttpMethod.Post, "https://api.twitter.com/oauth2/token");
req.Headers.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", b64);
req.Content = new FormUrlEncodedContent(new Dictionary<string, string>() {
    { "grant_type", "client_credentials" }
});
var token = "";
using (var res = await http.SendAsync(req))
{
    if (res.IsSuccessStatusCode)
        token = Regex.Match(await res.Content.ReadAsStringAsync(), "\"access_token\":\"([^\"]+)").Groups[1].Value;
}
http.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer ", token);

作为回应，我收到了这条消息：

{"errors":[{"code":99,"message":"Unable to verify your credentials","label":"authenticity_token_error"}]}