diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,7 @@
+2015-11-03 Rodrigo Setti <rodrigosetti@gmail.com>
+
+    * Updates cereal to 0.5.0.0
+
 2015-08-07 Will Sewell <will@pusher.com>
 
     * Add ObjectUInt constructor for messagepack unsigned ints
diff --git a/Data/MessagePack.hs b/Data/MessagePack.hs
--- a/Data/MessagePack.hs
+++ b/Data/MessagePack.hs
@@ -100,7 +100,7 @@
             | size < 0x10000 = putWord8 map16 >> putWord16be (fromIntegral size)
             | otherwise      = putWord8 map32 >> putWord32be (fromIntegral size)
 
-    put (ObjectExt t bytes) = header >> putWord8 (fromIntegral t) >> putByteString bytes
+    put (ObjectExt t bytes) = header2 >> putWord8 (fromIntegral t) >> putByteString bytes
       where
         size = BS.length bytes
         header
diff --git a/messagepack.cabal b/messagepack.cabal
--- a/messagepack.cabal
+++ b/messagepack.cabal
@@ -1,5 +1,5 @@
 name               : messagepack
-version            : 0.5.1
+version            : 0.5.2
 synopsis           : Serialize instance for Message Pack Object
 description        : Serialize instance for Message Pack Object
 homepage           : https://github.com/rodrigosetti/messagepack
diff --git a/tests/Main.hs b/tests/Main.hs
--- a/tests/Main.hs
+++ b/tests/Main.hs
@@ -33,12 +33,6 @@
     shrink (ObjectExt t s)  = map (ObjectExt t) $ shrink s
     shrink _                = []
 
-instance (Ord k, Arbitrary k, Arbitrary v) => Arbitrary (M.Map k v) where
-
-    arbitrary = M.fromList <$> arbitrary
-
-    shrink  = map M.fromList . shrink . M.toList
-
 instance Arbitrary BS.ByteString where
 
     arbitrary = BS.pack <$> arbitrary
