diff --git a/mercurial/posix.py b/mercurial/posix.py
--- a/mercurial/posix.py
+++ b/mercurial/posix.py
@@ -575,9 +575,9 @@
     if gid is None:
         gid = os.getgid()
     try:
-        return grp.getgrgid(gid)[0]
+        return pycompat.fsencode(grp.getgrgid(gid)[0])
     except KeyError:
-        return str(gid)
+        return pycompat.bytestr(gid)
 
 def groupmembers(name):
     """Return the list of members of the group with the given