Nullable parameter problem

class CExternalDevice
{        
      // MR_1
      internal CRawResponseData MakeRequest( RQHeader header, Byte[] cmdPayloadData, UInt32? expectedResponseSize)
      {
         // talk to external device not avail during test

         // ...

         // ultimately 

         return new CRawResponseData();
      }

      // MR_2
        internal CRawResponseData MakeRequest( RQHeader header, UInt32? expectedResponseSize)
        {
            return WriteCommandPacket(header, new Byte[] {}, expectedResponseSize);
        }

      // MR_3
        internal CRawResponseData MakeRequest( RQHeader header )
        {
            return WriteCommandPacket(header, new Byte[] {}, 0);
        }

      // MR_4
        internal CRawResponseData MakeRequest( RQHeader header, Byte[] data)
        {
            return WriteCommandPacket( header, data, 0);
        }
}


class CSomethingToTest
{
       private CFancyResponseData SimpleMakeFancyRequest( CATEGORY category, CODE cmd )
        {
            RQHeader header = new RQHeader( category, code );

            //Write packet with an unknown (null) response size
            CRawResponseData response = _myExternalDevice.MakeRequest( header, expectedResponseSize: null );

            return new CFancyResponseData( response );
        }

       private CRawResponseData SimpleMakeRequest( CATEGORY category, CODE cmd )
        {
            RQHeader header = new RQHeader( category, code );

            CRawResponseData response = _myExternalDevice.MakeRequest( header );

            return  response;
        }
}



[TestMethod( )]
public void MyTest()
{
         CSomethingToTest  testTarget = new CSomethingToTest();

         bool fakeDeviceConnected = true;

            CExternalDevice fakeExternalDevice = Isolate.Fake.Instance<CExternalDevice>( Members.ReturnRecursiveFakes, ConstructorWillBe.Ignored );
            Isolate.WhenCalled( ( ) => fakeExternalDevice.Connected( ) ).DoInstead( c =>
                {
                    return fakeDeviceConnected;
                }
            );

            testTarget._myExternalDevice = fakeExternalDevice;

            RQHeader dummyHeader = new RQHeader( 0, 0 );
            Byte[ ] dummyData = new byte[ 0 ];

         //  UInt32? dummyResponseSize = null;
         //  doing this makes ANON_FAKE_MR_1  throw an exception  "param cannot be null",

            UInt32? dummyResponseSize = 0;

         CRawResponseData simulatedResponse = new CRawResponseData();

            Isolate.WhenCalled( ( ) => fakeExternalDevice.MakeRequest( dummyHeader ) )
                                    .CallOriginal( );

            Isolate.WhenCalled( ( ) => fakeExternalDevice.MakeRequest( dummyHeader, dummyResponseSize ) )
                                    .CallOriginal( );

            Isolate.WhenCalled( ( ) => fakeExternalDevice.MakeRequest( dummyHeader, dummyData ) )
                                    .CallOriginal( );

         // ANON_FAKE_MR_1
            Isolate.WhenCalled( ( ) => fakeExternalDevice.MakeRequest( dummyHeader, dummyData, dummyResponseSize ) )
                                .DoInstead( callContext => 
                                    {
                                        return  simulatedResponse;
                                    }
            );

         ////////////////
         //  this calls MR_3, and then ANON_FAKE_MR_1, as expected
         testTarget.SimpleMakeRequest( 11, 22 );

         /////////////////
         //  this calls MR_2, and then MR_1  (ignores ANON_FAKE_MR_1)  NOT expected (or wanted)
         testTarget.SimpleMakeFancyRequest( 10, 20 );


}