diff --git a/Distribution/Simple/Utils.hs b/Distribution/Simple/Utils.hs
index 84343e8b6846b3f254ed3422a429b89bbccb260c..33ef40bd78af6ead1b8ef15488ed03b13232bfca 100644
--- a/Distribution/Simple/Utils.hs
+++ b/Distribution/Simple/Utils.hs
@@ -259,7 +259,11 @@ rawSystemStdout' verbosity path args = do
 
       -- fork off a thread to pull on (and discard) the stderr
       -- so if the process writes to stderr we do not block.
-      forkIO $ hGetContents errh >>= evaluate . length >> return ()
+      -- NB. do the hGetContents synchronously, otherwise the outer
+      -- bracket can exit before this thread has run, and hGetContents
+      -- will fail.
+      err <- hGetContents errh 
+      forkIO $ do evaluate (length err); return ()
 
       -- wait for all the output
       output <- hGetContents outh