Ambiguity error with higher order lists and type families

Cats.hs

{-# LANGUAGE KindSignatures, DataKinds, TypeFamilies, TypeOperators, GADTs, FlexibleInstances, FlexibleContexts #-}

module Foo where

data HList as where
    HNil :: HList '[]
    HCons :: a -> HList as -> HList (a ': as)

instance Show (HList '[]) where
	show _ = "[]"

instance (Show a, Show (HList as)) => Show (HList (a ': as)) where
    show (HCons a as) = show a ++ ":" ++ show as

first :: HList (a ': as) -> HList '[a]
first (HCons a _) = HCons a HNil

newtype Cat a = Cat a

type family MapCat (as :: [*]) :: [*] where
    MapCat '[] = '[]
    MapCat (a ': as) = Cat a ': MapCat as

type CatList cs = HList (MapCat cs)

first' :: CatList (a ': as) -> CatList '[a]
first' (HCons a _) = HCons a HNil

Couldn't match type ‘MapCat as0’ with ‘MapCat as’
NB: ‘MapCat’ is a type function, and may not be injective
The type variable ‘as0’ is ambiguous
Expected type: CatList (a : as) -> CatList '[a]
  Actual type: CatList (a : as0) -> CatList '[a]
In the ambiguity check for:
  forall a (as :: [*]). CatList (a : as) -> CatList '[a]
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature:
 first' :: CatList (a : as) -> CatList '[a]