Compaction failing on an existential type carrying a type class dict

CompactFail.hs

#!/usr/bin/env stack
-- stack --resolver nightly-2017-08-20 --install-ghc runghc --package compact
{-# LANGUAGE ExistentialQuantification #-}
module Main where

import Data.Compact

class SomeClass a where
  someId :: a -> a

instance SomeClass Int where
  someId = id

data Wrapper a = SomeClass a => Wrapper a

main :: IO ()
main = do
  _ <- compact $ Wrapper (1 :: Int)
  putStrLn "Will fail"