diff --git a/src/Data/OpenUnion/Internal.hs b/src/Data/OpenUnion/Internal.hs
index 2781ab9..ac73778 100644
--- a/src/Data/OpenUnion/Internal.hs
+++ b/src/Data/OpenUnion/Internal.hs
@@ -5,6 +5,7 @@
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE RoleAnnotations #-}
 
 -- |
 -- Module:       Data.OpenUnion.Internal
@@ -40,6 +41,8 @@ import Unsafe.Coerce (unsafeCoerce)
 data Union (r :: [Type -> Type]) a where
   Union :: {-# UNPACK #-} !Word -> t a -> Union r a
 
+type role Union nominal nominal
+
 -- | Takes a request of type @t :: * -> *@, and injects it into the 'Union'.
 --
 -- Summand is assigning a specified 'Word' value, which is a position in the