Adding Test Results Through API

I am trying to add test results via the API and am getting a (400): Bad Request error. Does anyone know what might be wrong with this code? It worked on the old API before it required login credentials.

using (var client = new WebClient())
            {
                string testResultID = "59";
                string BaseURI = "http://testrailurl/index.php?/api/v2/";
                string url = string.Format("{0}add_result/{1}", BaseURI, testResultId);

                string authInfo = "login@email.com:password";
                authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));
                client.Headers["Authorization"] = "Basic " + authInfo;

                client.Headers.Add("Accept: application/json");
                client.Headers.Add("Content-Type: application/x-www-form-urlencoded");

                var data = new NameValueCollection();
                data.Add("status_id", testStatus);
                data.Add("comment", comment);

                var response = client.UploadValues(url, "POST", data);
                var str = Encoding.Default.GetString(response);
            }