Swap not working as expected

I know I must be doing something wrong. This is a very simple test, but I'm having issues get the returned SPListItem Guid to be populated with the expected value. The UniqueId property for the returned SPListItem is always {000000...

Test code
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//Arrange
Guid testGuid = Guid.NewGuid();

SPListItem fakeListItem = Isolate.Fake.Instance<SPListItem>(Members.ReturnRecursiveFakes);
SPWeb fakeWeb = Isolate.Fake.Instance<SPWeb>(Members.ReturnRecursiveFakes);
Isolate.WhenCalled(() => fakeListItem.UniqueId).WillReturn(testGuid);
Isolate.Swap.NextInstance<SPListItem>().With(fakeListItem);

//Act
Registration registration = new Registration();
registration.Title = "Title";
registration.Status = "Status";

RegistrationRepository repository = new RegistrationRepository();

SPListItem listItemAdded = repository.AddNewRegistrationEntry(registration, fakeWeb);
//Assert
Assert.AreEqual(testGuid, listItemAdded.UniqueId);



Code Under Test
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public class RegistrationRepository
{
    private readonly string listName = "RegistrationRepository";

    public SPListItem AddNewRegistrationEntry(Registration registration, SPWeb web)
    {
        return AddListItem(registration, web);
    }

    private SPListItem AddListItem(Registration registration, SPWeb web)
    {
        if (web == null) throw new ArgumentNullException("web", "SPWeb instance cannot be null");

        SPListItem item = web.Lists[listName].Items.Add();

        item["Title"] = registration.Title;
        item["Status"] = registration.Status;

        return item;
    }
}

I'm very new TypeMock, so I realize that I'm probably missing something obvious. Any help is appreciated.