Entity Framework 4.0 AddObject() Method Encounter OptimisticConcurrencyException!!!!!!!

Question

I'm now using EF 4.0，and I Create a sample Table Named TableB,Table B‘s PK is (Id,YearMonth,Time),and Column ClientId Has been set as Identity ,As the following two Pics show

when I Add a new record , Exception occrued ,as the following pic show

And the full callstack is

   在 System.Data.Mapping.Update.Internal.UpdateTranslator.ValidateRowsAffected(Int64 rowsAffected, UpdateCommand source)
   在 System.Data.Mapping.Update.Internal.UpdateTranslator.Update(IEntityStateManager stateManager, IEntityAdapter adapter)
   在 System.Data.EntityClient.EntityAdapter.Update(IEntityStateManager entityCache)
   在 System.Data.Objects.ObjectContext.SaveChanges(SaveOptions options)
   在 System.Data.Objects.ObjectContext.SaveChanges()

If I set the CientId Column not a identity column，every thing works fine

Can anyone tell me why this problem happens? thanks in advance