panic on type family

Ghc panics on this module:

{-# LANGUAGE TypeFamilies #-}

type family Element c :: *

f :: Element x
f = undefined

------
ghc-6.8.0.20070921: panic! (the 'impossible' happened)
  (GHC version 6.8.0.20070921 for i386-unknown-linux):
        check_tau_type main:Main.Element{tc r5C} x{tv a5F} [tv]

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug