Cannot get Typeable for related data families

I am trying to get Typeable

for a related dataset like this:

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE TypeFamilies #-}
module Test where

import Data.Typeable

class Test a where
  data DTest a :: *

instance Test () where
  data DTest () = Foo
    deriving Typeable

      

        
        
        
      

    

But I am getting the following error, and it leaves me a bit puzzled:

[1 of 1] Compiling Test             ( test.hs, test.o )

test.hs:12:14:
    Cannot eta-reduce to an instance of form
      instance (...) => Typeable DTest
    In the data instance declaration for ‘DTest’

      

        
        
        
      

    

Who can tell what is going wrong here?