diff --git a/mercurial/windows.py b/mercurial/windows.py
--- a/mercurial/windows.py
+++ b/mercurial/windows.py
@@ -8,6 +8,7 @@
 from __future__ import absolute_import
 
 import errno
+import getpass
 import msvcrt
 import os
 import re
@@ -563,6 +564,8 @@
     """Return the name of the user with the given uid.
 
     If uid is None, return the name of the current user."""
+    if not uid:
+        return pycompat.fsencode(getpass.getuser())
     return None