Error in LINQ - New transaction is not allowed because there are other threads running in the session

Hi
I am not an experience in LINQ so far, and is having the following error:

New transaction is not allowed because there are other threads running in the session

My code:

--------------
foreach(var ReqDetail in (from row in Db.ReqDetail.With(LockHint.UpdLock)
                  where row.Company == Session.CompanyID && row.ReqNum == iReqNum
                  select new { row.PartNum, row.Class, row.LineDesc, row.IUM, row.TranType, row.JobNum, row.Number01, row.XOrderQty, row.ReqNum, row.ReqLine }      )  )
                  {
                        
                              RFQItem newRow = new RFQItem();
                              Db.RFQItem.Insert(newRow);
                              newRow.Company  = Session.CompanyID;
                              newRow.OpenItem      = true ;
                              newRow.RFQNum       = (int)iRFQNum ;
                              newRow.RFQLine =       j;                              //ReqDetail.ReqLine ;
                              newRow.PartNum  = ReqDetail.PartNum;
                              newRow.ClassID      = ReqDetail.Class;
                                          
                              newRow.LineDesc = ReqDetail.LineDesc ;
                              newRow.IUM      = ReqDetail.IUM ;
                              newRow.PUM      = ReqDetail.IUM ;
                  
                              if (string.Compare(ReqDetail.TranType ,"PUR-UKN",true)==0)
                              {
                                        newRow.ItemType = "UKN";
                              }
                              else if (string.Compare(ReqDetail.TranType ,"PUR-MTL",true)==0)
                              {
                                        newRow.ItemType = "MTL";
                              }
                              else
                              {
                                        newRow.ItemType = "STK";
                              }
                              if (string.Compare(ReqDetail.TranType ,"PUR-MTL",true)==0)
                              {
                                        newRow.JobNum = ReqDetail.JobNum;
                                        newRow.JobSeq = (int)ReqDetail.Number01;
                              }
                  
                              newRow.RequiredQty      = ReqDetail.XOrderQty;
                              newRow.PurchasingFactor      = 1 ;
                              newRow.PurchasingFactorDirection = "D" ;
                                    
                              newRow.RFQVendQuotes       = (int)iQuoteReq;
                              newRow.Number01            = ReqDetail.ReqNum ;
                              newRow.Number02            = ReqDetail.ReqLine ;
                  
                              Db.Validate();
---------------------

The code is pretty simple as it is looking for each line in Requisition detail table and trying to create a RFQ line based on each Requisition line data.

Please suggest - what is causing an error here ?

Note: I have disabled the child records update but still this is throwing the error.

regards