diff --git a/haskell/src/Data/MessagePack/Class.hs b/haskell/src/Data/MessagePack/Class.hs
index f50a4d82..34a7772f 100644
--- a/haskell/src/Data/MessagePack/Class.hs
+++ b/haskell/src/Data/MessagePack/Class.hs
@@ -46,6 +46,11 @@ instance OBJECT Object where
 fromObjectError :: String
 fromObjectError = "fromObject: cannot cast"
 
+instance OBJECT () where
+  toObject = const ObjectNil
+  fromObject ObjectNil = Right ()
+  fromObject _ = Left fromObjectError
+
 instance OBJECT Int where
   toObject = ObjectInteger
   fromObject (ObjectInteger n) = Right n