diff --git a/hgext/patchbomb.py b/hgext/patchbomb.py
--- a/hgext/patchbomb.py
+++ b/hgext/patchbomb.py
@@ -669,7 +669,8 @@
         start_time = util.makedate()
 
     def genmsgid(id):
-        return '<%s.%s@%s>' % (id[:20], int(start_time[0]), socket.getfqdn())
+        return '<%s.%d@%s>' % (id[:20], int(start_time[0]),
+                pycompat.sysbytes(socket.getfqdn()))
 
     # deprecated config: patchbomb.from
     sender = (opts.get('from') or ui.config('email', 'from') or