type families : couldn't match type "Elem a" against type "Elem a"

[guest]

I received the error

GHCi, version 6.8.0.20071012: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Test             ( Test.hs, interpreted )

Test.hs:17:27:
    Couldn't match expected type `Elem a'
           against inferred type `Elem a'
      Expected type: Elem a -> Maybe (Elem a) -> Maybe (Elem a)
      Inferred type: Elem a -> Maybe (Elem a) -> Maybe (Elem a)
    In the first argument of `foldr', namely `mf'
    In the second argument of `fromMaybe', namely
        `(foldr mf Nothing xs)'
Failed, modules loaded: none.

when running the following module in GHCi:

{-# LANGUAGE TypeFamilies #-}

module Test where

import qualified Prelude as P
import Prelude hiding (foldr, foldr1)

import Data.Maybe

type family Elem a

class Foldable a where
    foldr :: (Elem a -> b -> b) -> b -> a -> b

    foldr1 :: (Elem a -> Elem a -> Elem a) -> a -> Elem a
    foldr1 f xs = fromMaybe (error "foldr1: empty structure")
                    (foldr mf Nothing xs)
       where mf x Nothing  = Just x
             mf x (Just y) = Just (f x y)