Problem type checking with natural transformation in record type

Main.purs

module Main where

import Prelude
import Effect (Effect)
import Data.Foldable (fold)
import TryPureScript (h1, text, render)

data Nat m n = Nat (m ~> n)

data NatR m n = NatR { nat :: (m ~> n) }

morphS :: forall m n. Monad m => Monad n => (m ~> n) -> m Unit -> n Unit
morphS t fm = t fm

class (Monad m, Monad n) <= ClassNat nat m n where
  foo :: nat -> m Unit -> n Unit

instance classNat :: (Monad m, Monad n) => ClassNat (Nat m n) m n where
  foo (Nat nat) x = pure (morphS nat (pure unit)) x

-- instance classNatR :: (Monad m, Monad n) => ClassNat (NatR m n) m n where
--   foo (NatR { nat }) = pure (morphS nat (pure unit))
-- Error: Could not match type a4 with a1 
--   a4 rigid type variable bound at line 17, column 34-54...
-- Wat? Stays at line 17 even if I move the code around.

-- instance classNatR' :: (Monad m, Monad n) => ClassNat (NatR m n) m n where
--   foo (NatR { nat }) x = pure (morphS nat (pure unit)) x
-- Same Error: Could not match type a4 with a1

instance classNatR :: (Monad m, Monad n) => ClassNat (NatR m n) m n where
  foo (NatR n) = pure (morphS n.nat (pure unit))

main :: Effect Unit
main =
    render $ fold
      [ h1 (text "Try natural transformation in records!")
      ]