Why does the Snap example use Bytestring?

I'm following the Snap example project, but when I copied the example code, Stack gave me errors complaining about the use of writeBS and how it expects a ByteString but is given a [Char]. So I imported a ByteString library and tried to convert all of the strings to ByteStrings using the 'fromString' method, but Stack is giving me the same error, claiming that the Srings are still [Char]. Here's the doctored example code:

module Main where

import           Snap
import           Snap.Types
import           Snap.Util.FileServe
import           Control.Applicative
import           Heist
import qualified Data.ByteString.UTF8 as BU

main :: IO ()
main = quickHttpServe site

site :: Snap ()
site =
    ifTop (writeBS $ BU.fromString "hello world") <|>
    route [ ("foo", writeBS $ BU.fromString "bar")
          , ("echo/:echoparam", echoHandler)
          ] <|>
    dir "static" (serveDirectory ".")

echoHandler :: Snap ()
echoHandler = do
    param <- getParam "echoparam"
    maybe (writeBS $ BU.fromString "must specify echo/param in URL")
          writeBS $ BU.fromString param