diff --git a/mercurial/posix.py b/mercurial/posix.py
--- a/mercurial/posix.py
+++ b/mercurial/posix.py
@@ -541,9 +541,9 @@
     if uid is None:
         uid = os.getuid()
     try:
-        return pwd.getpwuid(uid)[0]
+        return pycompat.fsencode(pwd.getpwuid(uid)[0])
     except KeyError:
-        return str(uid)
+        return pycompat.bytestr(uid)
 
 def groupname(gid=None):
     """Return the name of the group with the given gid.