specified cast is not valid. linq

void insertDB()
       {
           string FileName = lblFileName.Text;
           string Extension = Path.GetExtension(FileName);
           string FolderPath = Server.MapPath(ConfigurationManager.AppSettings["FolderPath"]);
           string conStr = "";
           switch (Extension)
           {
               case ".xls": //Excel 97-03
                   conStr = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + FolderPath + FileName + ";Extended Properties=\"Excel 8.0;HDR=Yes;IMEX=2\"";
                   break;
               case ".xlsx": //Excel 07
                   conStr = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=" + FolderPath + FileName + ";Extended Properties=\"Excel 12.0 Xml;HDR=YES\"";
                   break;
           }
           try
           {
               using (var context = new LQTransAgentSeaFreightRateDataContext())
               {
                   string sql = string.Format("Select * FROM [{0}]" ,  ddlSheets.SelectedValue);
                   using (var myConnection = new OleDbConnection(conStr))
                   using (var myCommand = new OleDbCommand(sql, myConnection))
                   {
                       myConnection.Open();
                       var myReader = myCommand.ExecuteReader();
                       while (myReader.Read())
                       {
                           context.TB_TransAgentSeaFreightRates.InsertOnSubmit(new TB_TransAgentSeaFreightRate()
                           {
                               tASF_VCPOD = myReader.GetString(0).ToString(),
                               tASF_VCPOL = myReader.GetString(1).ToString(),
                               tASF_VCForwarder = myReader.GetString(2).ToString(),
                               tASF_VCForwarderReference = myReader.GetString(3).ToString(),
                               tASF_VCShippingLine = myReader.GetString(4).ToString(),
                               tASF_VCContainerType = myReader.GetString(5).ToString(),
                               tASF_VCContainerSize = myReader.GetString(6).ToString(),
                               tASF_DTEValidFrom = Convert.ToDateTime(myReader.GetString(7)),
                               tASF_DTEValidTo = Convert.ToDateTime(myReader.GetString(8)),
                               tASF_NUBasicRate = mobjGenlib.ConvertLong(myReader.GetString(9)),
                               tASF_NUPAF = mobjGenlib.ConvertLong(myReader.GetString(10)),
                               tASF_NUCAF = mobjGenlib.ConvertLong(myReader.GetString(11)),
                               tASF_NUPSS = mobjGenlib.ConvertLong(myReader.GetString(12)),
                               tASF_NUTotalAmount = mobjGenlib.ConvertLong(myReader.GetString(13)),
                               tASF_NUFreeDays = mobjGenlib.ConvertLong(myReader.GetString(14)),
                               tASF_VCCreditDays = myReader.GetString(15).ToString(),
                               tASF_VCNITDeposit = myReader.GetString(16).ToString(),
                               tASF_NUIsActive = 1,
                               tASF_mCMP_NUUniqueId = mobjGenlib.ConvertLong(TXTCompanyID.Text)
                           });
                       }
                   }

                   context.SubmitChanges();
               }
           }
           catch (Exception ex)
           {
               lblMessage.ForeColor = System.Drawing.Color.White;
               lblMessage.Text = ex.Message;
           }
       }