How do I create a functor instance for a type with two arguments, where both arguments have to be the same type?

I have almost been able to create a valid functor instance for the type Pair. Problem is, Pair takes two arguments both of the same type, so when I write

fmap f (Pair a a') = Pair a (f a')

I cannot guarantee that the result is a valid Pair, since (f a') might be any type.

What is the Haskell way of ensuring this constraint?

import Test.QuickCheck
import Test.QuickCheck.Function

data Pair a = Pair a a deriving (Eq, Show)

instance Functor Pair where
  fmap f (Pair a a') = Pair a (f a')

-- stuff below just related to quickchecking that functor instance is valid
main = quickCheck (functorCompose' :: PFC)

type P2P = Fun Int Int

type PFC = (Pair Int) -> P2P -> P2P -> Bool

instance (Arbitrary a) => Arbitrary (Pair a) where
  arbitrary = do
    a <- arbitrary
    b <- arbitrary
    return (Pair a b)

functorIdentity :: (Functor f, Eq (f a)) => f a -> Bool
functorIdentity f = fmap id f == f

functorCompose :: (Eq (f c), Functor f) => (a -> b) -> (b -> c) -> f a -> Bool
functorCompose f g x = (fmap g (fmap f x)) == (fmap (g . f) x)

functorCompose' :: (Eq (f c), Functor f) => f a -> Fun a b -> Fun b c -> Bool
functorCompose' x (Fun _ f) (Fun _ g) = (fmap (g . f) x) == (fmap g . fmap f $ x)

Here is the error message I get, btw:

chap16/functor_pair.hs:22:29: Couldn't match expected type ‘b’ with actual type ‘a’ …
      ‘a’ is a rigid type variable bound by
          the type signature for fmap :: (a -> b) -> Pair a -> Pair b
          at /Users/ebs/code/haskell/book/chap16/functor_pair.hs:22:3
      ‘b’ is a rigid type variable bound by
          the type signature for fmap :: (a -> b) -> Pair a -> Pair b
          at /Users/ebs/code/haskell/book/chap16/functor_pair.hs:22:3
    Relevant bindings include
      a' :: a
        (bound at /Users/ebs/code/haskell/book/chap16/functor_pair.hs:22:18)
      a :: a
        (bound at /Users/ebs/code/haskell/book/chap16/functor_pair.hs:22:16)
      f :: a -> b
        (bound at /Users/ebs/code/haskell/book/chap16/functor_pair.hs:22:8)
      fmap :: (a -> b) -> Pair a -> Pair b
        (bound at /Users/ebs/code/haskell/book/chap16/functor_pair.hs:22:3)
    In the first argument of ‘Pair’, namely ‘a’
    In the expression: Pair a (f a')
Compilation failed.

(This is an exercise from http://haskellbook.com/)