diff --git a/mercurial/posix.py b/mercurial/posix.py
--- a/mercurial/posix.py
+++ b/mercurial/posix.py
@@ -130,7 +130,7 @@
         # switch link to file
         data = os.readlink(f)
         unlink(f)
-        fp = open(f, "w")
+        fp = open(f, "wb")
         fp.write(data)
         fp.close()
         s = 0o666 & ~umask # avoid restatting for chmod