System.Timeout.timeout throws <<timeout>>

Summary

System.Timeout.timeout throws a Timeout exception to me, instead of returning Nothing as documented.

Steps to reproduce

Load this file:

{-# LANGUAGE ScopedTypeVariables #-}

module Bug where

import Control.Exception
import System.Timeout
import Data.Maybe

data UserCodeResult a
  = Timeout
  | Exception String
  | Success a
  deriving Show

userCode :: IO a -> IO (UserCodeResult a)
userCode action = fmap (fromMaybe Timeout) $
                  timeout numMicroseconds $ do
                    result <- action
                    forced <- evaluate result
                    pure (Success forced)
                  `catch` \ (e :: SomeException) -> pure (Exception (displayException e))
  where
    numMicroseconds = 1 * 10^6

fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

and then run userCode (return $ fib 1000).

Expected behavior

I want it to return Timeout (because timeout should return Nothing). But instead it returns Exception "<<timeout>>", indicating a thrown exception.

Environment

Tested on GHC 8.6.1, on Mac OS 10.13.5.