/orders/cart/v1 - 500 Internal Server Error

public object AddShoppingCartItem(int modelID, int materialID, int quantity = 1)
        {
            OAuthRequest client = new OAuthRequest()
            {
                Method = "POST",
                Type = OAuthRequestType.ProtectedResource,
                SignatureMethod = OAuthSignatureMethod.HmacSha1,
                ConsumerKey = this.consumerKey,
                ConsumerSecret = this.consumerSecret,
                RequestUrl = "https://api.shapeways.com/orders/cart/v1", 
                Version = "1.0a",
                Realm = "shapeways.com",
                TokenSecret = this.OAuthSecret,
                Token = this.OAuthToken
            };
            string auth = client.GetAuthorizationQuery();
            string requestUrl = client.RequestUrl + "?" + auth;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(requestUrl);
            request.ContentType = "application/json; charset=utf-8";
            request.Method = "POST";

            CartItem cartItem = new CartItem
            {
                materialId = materialID,
                modelId = modelID,
                quantity = quantity
            };
            
            using(StreamWriter writer = new StreamWriter(request.GetRequestStream()))
            {
                string json = JsonConvert.SerializeObject(cartItem);
                writer.Write(json);
                writer.Flush();
                writer.Close();

            }

            

            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            string content = new StreamReader(response.GetResponseStream()).ReadToEnd();
            return JsonConvert.DeserializeObject(content);


        }

        private object MakeShapewaysPostRequest(string url, string json)
        {
            OAuthRequest client = new OAuthRequest()
            {
                Method = "POST",
                Type = OAuthRequestType.ProtectedResource,
                SignatureMethod = OAuthSignatureMethod.HmacSha1,
                ConsumerKey = this.consumerKey,
                ConsumerSecret = this.consumerSecret,
                RequestUrl = url, 
                Version = "1.0a",
                Realm = "shapeways.com",
                TokenSecret = this.OAuthSecret,
                Token = this.OAuthToken
            };
            string auth = client.GetAuthorizationQuery();
            string requestUrl = client.RequestUrl + "?" + auth;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(requestUrl);
            request.ContentType = "application/json; charset=utf-8";
            request.Method = "POST";

            using (StreamWriter writer = new StreamWriter(request.GetRequestStream()))
            {
                writer.Write(json);
                writer.Flush();
                writer.Close();
            }

            HttpWebResponse response = null;

            try
            {
                response = (HttpWebResponse)request.GetResponse();
            }
            catch (WebException ex)
            {
                // Shapeways returns error 500 on any processing issues.
                // ASP throws exception on error 500. To get around this,
                // wrap in try/catch to get error response.
                if (ex.Response != null)
                {
                    response = (HttpWebResponse)ex.Response;
                }
            }

            string content = new StreamReader(response.GetResponseStream()).ReadToEnd();
            return JsonConvert.DeserializeObject(content);
        }