Some properties faking problem

public interface Parent
    {
        InternalProperty MainNternalProperty { get; set; }
    }

 public interface InternalProperty
    {
        ObservableCollection<Parent> Derives { get; }
    }

 public class Derived : Parent
    {
        private InternalProperty _mainNternalProperty;

        public InternalProperty MainNternalProperty
        {
            get { return _mainNternalProperty; }
            set
            {
                _mainNternalProperty = value;
                value.Derives.Add(this);

            }
        }
    }

I want to be sure that the Add method was called. so the test is: 
  [TestClass]
    public class ObservTests
    {
        private Derived _derived;

        [TestMethod]
        public void ObservTest()
        {
            // Arrange
            _derived = Isolate.Fake.Instance<Derived>(Members.CallOriginal);

            InternalProperty nternalProperty = Isolate.Fake.Instance<InternalProperty>();
            Isolate.WhenCalled(() => nternalProperty.Derives).WillReturn(Isolate.Fake.Instance<ObservableCollection<Parent>>());

            // Act
            _derived.MainNternalProperty = nternalProperty;

            // Assert
            Isolate.WhenCalled(() => nternalProperty.Derives.Add(_derived));

        }
    }