Problem putting JSON to riak via Haskell bindings

riak-put-json.hs

{-# LANGUAGE ScopedTypeVariables, OverloadedStrings #-}

import Network.Riak
import qualified Network.Riak.JSON as RiakJSON
import Network.Riak.Connection.Pool
import qualified Data.ByteString.Lazy.Char8 as B
import Control.Applicative
import qualified Data.Aeson as JSON

testPut :: IO ()
testPut = do
  pool <- create defaultClient 1 1 2
  withConnection pool myPut >>= \(_,vclock) -> print vclock
   where
    myPut conn = do
     (bs::B.ByteString) <- (JSON.encode . B.pack) <$> readFile "json.dat"
     RiakJSON.put conn "bucketFoo" "keyFoo" Nothing bs Default Default

{- ghci complaint
*Main> testPut
*** Exception: Riak type error (Network.Riak.Value.convert): 1 values failed conversion: [Content {value = "\"\\\"{\\\\n     \\\\\\\"firstName\\\\\\\": \\\\\\\"John\\\\\\\",\\\\n     \\\\\\\"lastName\\\\\\\": \\\\\\\"Smith\\\\\\\"    \\\\n }\\\\n\\\"\"", content_type = Just "application/json", charset = Nothing, content_encoding = Nothing, vtag = Just "gokFoPe2aLYM3QEiBYNyc", links = fromList [], last_mod = Just 1356195100, last_mod_usecs = Just 148285, usermeta = fromList [], indexes = fromList []}]
-}

{- json.dat file
{
     "firstName": "John",
     "lastName": "Smith"    
 }
-}