diff --git a/dns/query.py b/dns/query.py

index bfecd43..19b9fbb 100644

--- a/dns/query.py

+++ b/dns/query.py

@@ -467,7 +467,8 @@ def xfr(where, zone, rdtype=dns.rdatatype.AXFR, rdclass=dns.rdataclass.IN,

     first = True

     while not done:

         mexpiration = _compute_expiration(timeout)

-        if mexpiration is None or mexpiration > expiration:

+        if mexpiration is None or \

+           (expiration is not None and mexpiration > expiration):

             mexpiration = expiration

         if use_udp:

             _wait_for_readable(s, expiration)