Error GHC-91510: illegal polymorphic type

I’m working on constrained categories, and I want to write the following:

type Category :: (k -> k -> Type) -> Constraint
class Category cat where
  type Objects cat (x :: k) :: Constraint
  type Objects cat x = ()
  id :: (Objects cat x) => cat x x
  (.) ::
    (Objects cat x, Objects cat y, Objects cat z) => cat y z -> cat x y -> cat x z

instance Category (->) where
  id :: x -> x
  id x = x
  (.) :: (y -> z) -> (x -> y) -> x -> z
  y_z . x_y = \x -> y_z (x_y x)

That’s all well and good, but then I encountered an undocumented error while trying to extend this to natural transformations:

type Transform ::
  (k -> k -> Type) ->
  (j -> k) ->
  (j -> k) ->
  Type
newtype Transform cat f g = Transform
  {transform :: forall x. cat (f x) (g x)}

instance (Category cat) => Category (Transform cat) where
  type Objects (Transform cat) f = forall x. Objects cat (f x) -- GHC-91510
  id :: Transform cat f f
  id = Transform id
  (.) :: Transform cat g h -> Transform cat f g -> Transform cat f h
  Transform g_h . Transform f_g = Transform (g_h . f_g)

The error text is

• Illegal polymorphic type: forall (x :: k2). Objects cat (f x)
• In the type instance declaration for ‘Objects’
  In the instance declaration for ‘Category (Transform cat)’

Why is this polymorphic type illegal? It’s just a quantified constraint. What can be done to legalize it?