Type error when compiling ST-monad-like code

Compiling the module below works fine in GHC 6.4.2. In GHC 6.6 and 6.6.1, it gives a type error.

/Koen

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

import Control.Monad.ST
import Data.STRef

newtype M s a =
  MkM (STRef s Int -> ST s a)

runM :: (forall s . M s a) -> a
runM mm =
  runST (
    do ref <- newSTRef 0
       m ref
  )
 where
  MkM m = mm

-- the instance declaration and function definition 
-- of "inc" are just here for giving context; 
-- removing them still makes runM not type check in GHC 6.6

instance Monad (M s) where
  return x =
    MkM (\_ -> return x)

  MkM m >>= k =
    MkM (\ref ->
      do x <- m ref
         let MkM m' = k x
         m' ref
    )

inc :: M s Int
inc = MkM (\ref ->
  do n <- readSTRef ref
     writeSTRef ref (n+1)
     return n
  )