Occurs check error from equality constraint

I was writing that manipulates witnesses for equality constraints, and saw some occurs check errors for equations matching my constraints. (This is with ghc-6.9.20071025).

Bug.hs:19:31:
    Occurs check: cannot construct the infinite type: n = Sum Z n
    In the pattern: Refl
    In a case alternative: Refl -> Refl
    In the expression: case zerol n of Refl -> Refl

Here is the program producing that error:

{-# OPTIONS -fglasgow-exts #-}
module Bug where

data Z
data S a

type family Sum n m
type instance Sum n Z = n
type instance Sum n (S m) = S (Sum n m)

data Nat n where
  NZ :: Nat Z
  NS :: (S n ~ sn) => Nat n -> Nat sn

data EQ a b = forall q . (a ~ b) => Refl

zerol :: Nat n -> EQ n (Sum Z n)
zerol NZ = Refl
zerol (NS n) = case zerol n of Refl -> Refl

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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