NullReferenceException when updating/adding fields on an ite

I have recently upgraded to Typemock 5.4.5 from 5.4.4 and now a number of our tests fail with a nullreferenceexception.

The following MSTest Test Case highlights the problem and represent something that our code is doing:

The following test case throws the exception when *INSERT_INT* is greater than 2 i.e. more than 2 updates are made on the SPListItem:

**CODE**
SPWeb fakeWeb = Isolate.Fake.Instance<SPWeb>(Members.ReturnRecursiveFakes);
SPListItem fakeListItem = Isolate.Fake.Instance<SPListItem>(Members.ReturnRecursiveFakes);

Isolate.Swap.NextInstance<SPListItem>().With(fakeListItem);
SPList list = fakeWeb.getList("somedummylist");
SPListItem newItem = list.Items.Add();


for(int i =0; i < *INSERT_INT*; i++)
{
string fieldId = "testfield" + i;
newItem[fieldId] = "asdada";
}

***CODE***

Has the API changed in 5.4.5? or is it just something that I am doing that was illegal and now enforced in 5.4.5?