Safecopy transition from string to bytestring

I am trying to use safecopy library haskell, but when I try to cast a string to a byte string, the last 4 characters are lost and 4 '\ nULL' characters are appended to the string:

{-# LANGUAGE TemplateHaskell, DeriveDataTypeable, TypeFamilies #-}
import Data.SafeCopy
import Data.Acid
import Data.Typeable
import Control.Monad.State.Class
import Control.Monad.Reader.Class

data T = T { str :: String }
         deriving (Show, Typeable)

getT :: Query T String
getT = fmap str ask

setT :: String -> Update T ()
setT str = put $ T str

deriveSafeCopy 0 'base  ''T

makeAcidic ''T ['setT, 'getT]

main :: IO ()
main = do
  state <- openLocalState (T "string set with default")
  update state (SetT "string set with SetT")
  str <- query state GetT
  putStrLn str

      

        
        
        
      

    

This outputs:, string set with SetT

but after that after the next modification version:

{-# LANGUAGE TemplateHaskell, DeriveDataTypeable, TypeFamilies #-}
import Data.SafeCopy
import Data.Acid
import Data.Typeable
import Control.Monad.State.Class
import Control.Monad.Reader.Class
import Data.ByteString.Char8 as B

data T_v0 = T_v0 String
         deriving (Show, Typeable)
deriveSafeCopy 0 'base  ''T_v0

data T = T { str :: B.ByteString }
         deriving (Show, Typeable)
deriveSafeCopy 1 'extension ''T

instance Migrate T where
  type MigrateFrom T = T_v0
  migrate (T_v0 str) = T $ B.pack str

getT :: Query T B.ByteString
getT = fmap str ask

setT :: B.ByteString -> Update T ()
setT str = put $ T str
makeAcidic ''T ['setT, 'getT]

main :: IO ()
main = do
  state <- openLocalState (T $ B.pack "bytestring set with default")
  str <- query state GetT
  print str

      

        
        
        
      

    

it displays: "\NUL\NUL\NUL\NULstring set with "

. I have no idea why this is happening. Did I do something wrong during the migration stage? I tried to get as close as possible to the example code . Does anyone know why this is happening?

PS: Sorry for the large amount of code, but I couldn't think of a better way to convey the problem.