ghc.exe: panic! (the `impossible' happened, GHC version 6.4)

The compiler has asked me to send a bug report...so here it is.  This
is with the Windows distribution of GHC 6.4.

Compiling the following code:

import Graphics.UI.ObjectIO hiding (Menu)

class CScreen a where
      updateScreen :: a -> a
      drawScreen :: a -> Draw ()

data ScreenStack e l = EmptyScreenStack | ScreenStack (CScreen e) l


Produces the following message:

ghc -package objectio t.hs
ghc.exe: panic! (the `impossible' happened, GHC version 6.4):
        ds_app_type Main.CScreen{tc r1TC} [e{tv a1V4}]

Please report it as a compiler bug to glasgow-haskell-bugs@xxxxxxxxxxx,
or http://sourceforge.net/projects/ghc/.


I expect the code isn't correct, but I guess the compiler should give
a sensible error messge.

Thanks,
Damyan.