diff --git a/mercurial/worker.py b/mercurial/worker.py
--- a/mercurial/worker.py
+++ b/mercurial/worker.py
@@ -226,7 +226,7 @@
     selector = selectors.DefaultSelector()
     for rfd, wfd in pipes:
         os.close(wfd)
-        selector.register(os.fdopen(rfd, 'rb', 0), selectors.EVENT_READ)
+        selector.register(os.fdopen(rfd, 'rb'), selectors.EVENT_READ)
 
     def cleanup():
         signal.signal(signal.SIGINT, oldhandler)