Internal Server Error 500 - type validation in VSTS.. can somebody help solve the error

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Xml;
using System.Xml.Linq;
using System.Data;
using System.Data.OleDb;
using Microsoft.VisualStudio.TestTools.WebTesting;


namespace datadrivenlib
{
    public class Class1
    {
    }
    public class AddNumbers_WTRP : WebTestRequestPlugin
    {
        public override void PreRequest(object sender, PreRequestEventArgs e)
        {

        }
        public override void PostRequest(object sender, PostRequestEventArgs e)
        {
           
        }
    }
    public class AddNumbers_WTP : WebTestPlugin
    {
        public override void PostWebTest(object sender, PostWebTestEventArgs e)
        {
            //Out_Sum

            try
            {
               
                string StrOutputFile = @"Sum_Res.csv";
                
            string a = e.WebTest.Context["DataSource_CSV.Sum_Input#csv.A"].ToString();
                string b = e.WebTest.Context["DataSource_CSV.Sum_Input#csv.B"].ToString();

                //Output Parameters
                string sum = string.Empty;

                if (e.WebTest.Outcome != Outcome.Fail)
                {
                    sum = e.WebTest.Context["Out_Sum"].ToString();
                }
                else
                {
                    e.WebTest.Outcome = Outcome.Fail;
                }
                string InsertCmd = string.Format("Insert into {0} (a,b,sum) values('" + a + "," + b + "," + sum + ")", StrOutputFile);
             


            }

            catch (System.Data.OleDb.OleDbException ex)
            {
                e.WebTest.Outcome = Outcome.Fail;
            }
        }


    }

}

- <soap:envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
- <soap:body>
- <soap:fault>
  <faultcode>soap:Client</faultcode> 
  <faultstring>System.Web.Services.Protocols.SoapException: Server was unable to read request. ---> System.InvalidOperationException: There is an error in XML document (5, 14). ---> System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at System.Xml.XmlConvert.ToInt32(String s) at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReader1.Read1_addition() at Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer.Deserialize(XmlSerializationReader reader) at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events) --- End of inner exception stack trace --- at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events) at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle) at System.Web.Services.Protocols.SoapServerProtocol.ReadParameters() --- End of inner exception stack trace --- at System.Web.Services.Protocols.SoapServerProtocol.ReadParameters() at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()</faultstring> 
  <detail /> 
  </soap:fault>
  </soap:body>
  </soap:envelope>

a	b
12	13
14	15
16	17
18	19
10	20