Error When Get Result type DataTable form Web Service(ASMX)

I have problem in my web app with asmx web service
when i return Data type of DataTable include column (Int) give my error and this message error (-1 is not a valid value for int64)
befor is work fine but after i install new visaul studio 2015 not working my code is :
in web service (ASMX) :
[WebMethod]
public DataTable allchant(string password)
{
tablechantTableAdapter chanttable = new tablechantTableAdapter();
DataTable dt = new DataTable();

if(password=="thismyapp")

        dt = chanttable.GetData();

    return dt;
}

////////////////////////////////////////////////////////////////////////////////////////////
in my web app :
private void Btn2_Click(object sender, System.EventArgs e)
{
songservice.ZawamelService ws = new songservice.ZawamelService();

        ws.allchantAsync("thismyapp");
        ws.allchantCompleted +=Ws_allchantCompleted;}privatevoidWs_allchantCompleted(object sender, songservice.allchantCompletedEventArgs e){DataTable dt =newDataTable();
        dt = e.Result;}