linq - Row not found or changed

trouble:

var daskTeklif = _context.tblDaskTeklifs.SingleOrDefault(dt => dt.teklif_id == TeklifTkID);
daskTeklif.SirketPoliceNo = policeNo;
_context.SubmitChanges();

throws an exception: System.Data.Linq.ChangeConflictException: Row not found or changed.

solution:

creating a new datacontext:

var context = new DataClassesDataContext();
daskTeklif = context.tblDaskTeklifs.SingleOrDefault(dt => dt.teklif_id == TeklifTkID);
daskTeklif.sirketPoliceNo = policeNo;
context.SubmitChanges();