hostComponent is null in a skin who's type doesn't match but no compile or runtime error is given

This was found in a nightly build so I didn't find it in the version list above. It was build
4.0.0.11566 on Nov 8 2009
Steps to reproduce:
1.Get attached project
2.Run and see console output saying hostComponent is not null
3.Change the type in BugRepo.mxml(main) from components:MyButton to s:SkinnableContainer and run, see the trace showing the hostComponent is null

Actual Results:
Wrong class for a skin is accepted and no runtime or compile error is given to indicate this is a problem. Gets set to null.

Expected Results:
Ideally compile warning\error but else some debug output on console.

Workaround (if any):
Don't use the wrong component