Setting NULL into a column

   I can't believe I'm not getting this.

I've got a table ContractFunds, with a nullable column FundSource, which refers to a FundSource table.  The ServerModel and DomainModel entities exist and have no interesting bits.

I create ContractFund record by hand, having NULL in the FundSource column, and having retrieved this record via IdeaBlade, the FundSource property is (correctly) a NullEntity (IsNullEntity == true).  I continue by retrieving an appropriate FundSource, set it into the ContractFund, save, and the record in the database is updated.  Now I want to undo that last action.  I retrieve the ContractFund, set the FundSource to null, which appears to have the desired effect, but after the save the record in the database still has a value in the FundSource column.  I know the save worked because the housekeeping columns have updated.  When I crank up the Profiler, the update statement shows the housekeeping columns being updated, but the changes to the FundSource column aren't there.

Here's the code:

     Contract testContract = Contract.Get(em, 7);

     ContractFund fund = testContract.ContractFunds[0];

     FundSource fundSource = fund.FundSource;

     if (fundSource.EntityAspect.IsNullEntity)

     {

          FundSource newSource = FundSource.Get(em, "FAA");

          fund.FundSource = newSource;

     }

     else

     {

          fund.FundSource = null;

     }

     if (em.HasChanges())

          em.SaveChanges();

What am I missing?