Found "ghc-6.4: panic!"

I have just gotten:

ghc-6.4: panic! (the `impossible' happened, GHC version 6.4):
        Unify.unifyTauTyLists: mismatched type lists!

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

from ghci. The code I loaded is:

module ProgramOptions
    where

-- newtype Command n = Command [Option n]
--                   deriving (Show)
-- 
-- data Option n = Option { keys :: [String],
--                          value :: Value n,
--                          description :: String }
--               deriving (Show)
data Option = Option [String] Value String

data Num n => Value n = Boolean Bool
                      | Numeric n
                      | String String
instance Num n => Show (Value n) where
    show (Boolean b) = show b
    show (Numeric n) = show n
    show (String s) = show s

and the whole session was:

$ ghci
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 6.4, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base-1.0 ... linking ... done.
Prelude> :l ProgramOptions
Compiling ProgramOptions   ( ProgramOptions.hs, interpreted )
Ok, modules loaded: ProgramOptions.
*ProgramOptions> Option ["n", "name"] (String "Pupeno") "Name"
ghc-6.4: panic! (the `impossible' happened, GHC version 6.4):
        Unify.unifyTauTyLists: mismatched type lists!

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


*ProgramOptions>

I am reporting mainly because the interpreter asked for it, I am very new to Haskell.

Trac metadata

Trac field	Value
Version	6.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture