Levity polymorphism panic

I imagine I have made a mistake, but GHC shouldn't panic!

{-# language TypeInType, ScopedTypeVariables, MagicHash #-}

import GHC.Exts (TYPE, Proxy#, proxy#)

fold :: forall rep a (r :: TYPE rep).
          (r -> a -> Proxy# r -> r) -> (Proxy# r -> r) -> [a] -> r
fold f k [] = k proxy#
fold f k (x : xs) = fold f (f (k proxy#) x) xs

This gives me

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.5.20171211 for x86_64-unknown-linux):
        splitFunTy
  ()
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:921:30 in ghc:Type

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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