[Haskell-beginners] Strange "Not in scope" error message

I have a file with the following relevant lines: import qualified Control.Concurrent as C ... isCancelled = C.isEmptyMvar . stopConcurrentProcess Trying to compile this or run it in ghci yields a puzzling error message: Not in scope: `C.isEmptyMvar' Perhaps you meant one of these: `C.isEmptyMVar' (imported from Control.Concurrent), `C.isEmptyChan' (imported from Control.Concurrent) Any idea what could be going wrong?