Commented Issue: LongListSelector not working with IGrouping [7791]

I tried to bind a IEnumerable<IGrouping<DateTime, MyClass>> to LLS.ItemsSource. With some sample data, LLS looks fine in Expression Blend designer. The group title was bound to IGrouping.Key. But when I run the app in emulator, the group title has gone, just like nothing there. I have value converter applied to that binding, and I've set a breaking point inside the Convert method. But it seems that the Convert was not invoked at all. When I create a Group class derived from List<MyClass> with an addition Key property, and bind IEnumerable<Group> to LLS.ItemsSource, everything's OK. So LLS doesn't work with IGrouping? Is it a binding issue?
Comments: ** Comment from web user: Licantrop0 **

I have this problem too! I'm binding my LLS with a Linq.Lookup<char, MyViewModel> (that is the correct implementation for IEnumerable<IGrouping<DateTime, MyViewModel>>) and the key of the single group is correctly displayed in Visual Studio and blend, but not on the phone!