Problem with hugs, concurrency and unsafeInterleaveIO (maybe a bug?)

I am trying to lazily wait an MVar in hugs, in conjunction with concurrent haskell: ----- import Concurrent import IOExts f = do v <- newEmptyMVar c <- getContents forkIO (putMVar v (head c)) r <- unsafeInterleaveIO (takeMVar v) return v f2 = f >>= unsafeInterleaveIO . takeMVar main1 = f >>= takeMVar >>= print main2 = f2 >>= print main3 = f2 >>= (\ x -> yield >> print x) ----- main1 and main3 work flawlessly, but main2 aborts with "no more threads to run". I think that main2 should work, because in evaluating the result of f2, the main thread should suspend and yield. Is there somebody who wants to enlighten me? Vincenzo