This is the code: {-# OPTIONS -fglasgow-exts #-} import Foreign type X u = Ptr () foreign import ccall bla :: (forall u. X u) -> IO () I know of course that I must not use fancy types in foreign imports. I forgot for a moment and instead of an error message got: ben@sarun> ghci Bug3 GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( Bug3.hs, interpreted ) ghc: panic! (the 'impossible' happened) (GHC version 6.10.1 for i386-unknown-linux): unboxArg: Bug3.hs:4:0-51 forall u{tv aht} [tv]. main:Main.X{tc rhp} u{tv aht} [tv] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Cheers Ben