Is this a bug in multithreading code?

[Nick Name]

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