ResourcesWithContT.hs

main :: IO ()
main = do
  evalContT $ do
    a <- ContT $ withFile "a.txt" ReadMode
    b <- ContT $ withFile "b.txt" ReadMode
    aContents <- liftIO $ hGetContents a
    bContents <- liftIO $ hGetContents b
    liftIO . putStrLn $ "contents of a: " <> show aContents
    liftIO . putStrLn $ "contents of b: " <> show bContents

When the file a.txt exists, but the file b.txt does not exist, the following error is reported when running the above:

my-exe: a.txt: withFile: does not exist (No such file or directory)

It would be less confusing if that error said that b.txt did not exists, instead of complaining about a.txt. What am I missing?