diff --git a/mercurial/posix.py b/mercurial/posix.py
--- a/mercurial/posix.py
+++ b/mercurial/posix.py
@@ -543,7 +543,7 @@
     try:
         return pycompat.fsencode(pwd.getpwuid(uid)[0])
     except KeyError:
-        return pycompat.bytestr(uid)
+        return b'%d' % uid
 
 def groupname(gid=None):
     """Return the name of the group with the given gid.