Problem mocking an index accessor

I'm working with the Autodesk Inventor SDK Com Interop library.

I've got a test set up where I need to return a mocked object from an indexed accessor but it gives me null instead of what I set it up to return:

            var drawing = Isolate.Fake.Instance<DrawingDocument>();
            var sheets = Isolate.Fake.Instance<Sheets>();
            var sheet = Isolate.Fake.Instance<Sheet>();
            Isolate.WhenCalled(() => sheets[null]).WillReturn(sheet);
            var testSheet = drawing.Sheets[1];
            testSheet.Activate();

that last line throws an exception because testSheet is null. What am I doing wrong with this?