[Haskell-cafe] ghc: Var/Type length mismatch message - what does it mean?

consider the following types (from the paper: "Stream Fusion From Lists to Streams to Nothing at All")

data Stream a = forall s. Stream ( s ->(Step a s)) s

data Step a s = Done | Yield a s | Skip s

an instance of this data type is:

stream0 :: Stream () stream0 = Stream (\ s -> Yield () s) ()

now consider:

data StreamM m a = forall s. StreamM ( s ->m (Step a s)) s

and the following instance:

stream0IO :: StreamM IO () stream0IO = StreamM (\ s -> return (Yield () s)) ()

loads ok, but as soon as we introduce the following constraint:

data Monad m => StreamM' m a = forall s. StreamM' ( s ->m (Step a s)) s

stream0IO' :: StreamM' IO () stream0IO' = StreamM' (\ s -> return (Yield () s)) ()

we get the following message when we load into ghci (6.8.2) Var/Type length mismatch: [] [base:GHC.Base.(){(w) tc 40}] is this a bug? if not, what is this message telling us ?