A forkIO'ed thread that is reading from the TTY cannot be killed?

example.hs

#!/usr/bin/env stack
-- stack script --resolver lts --package unix
module Main where

import Control.Concurrent
import Control.Concurrent.MVar
import System.Posix

main :: IO ()
main = do
    sync <- newEmptyMVar
    threadId <- forkIO $ putMVar sync () >> fdRead stdInput 1 >>= print
    takeMVar sync
    putStrLn "kill?"
    killThread threadId
    putStrLn "kill!"