Is this a bug in multithreading code?

Hi all, does someone know why this program works ------------------------ import Concurrent loopM = sequence_ . repeat main = do v <- newMVar 0 forkIO (loopM (modifyMVar_ v (return . (1+)))) -- forkIO (loopM (modifyMVar_ v (return . (1+)))) loopM (readMVar v >>= print) ------------------------- but uncommenting the commented line makes the program fail with "stack overflow" ? Thanks for help Vincenzo