diff --git a/mercurial/hgweb/server.py b/mercurial/hgweb/server.py
--- a/mercurial/hgweb/server.py
+++ b/mercurial/hgweb/server.py
@@ -273,7 +273,7 @@
 
 def openlog(opt, default):
     if opt and opt != '-':
-        return open(opt, 'a')
+        return open(opt, 'ab')
     return default
 
 class MercurialHTTPServer(_mixin, httpservermod.httpserver, object):