Typemock Isolator 6 SPListItem problem with string

Hi,

I've been having some problem with using TypeMock.

SPListItem item = Isolate.Fake.Instance<SPListItem>();

item["KeyIndexMC4"] = "A";

string fieldRef = string.Concat("KeyIndex", "MC4");
item[fieldRef] = "B";

Assert.AreEqual("B", item["KeyIndexMC4"]);

The assertion will always return fail result.
I'm hoping I can find out if this is a bug.

Thanks in advance!