Are you sure about "if not san and not dnsnames"? It is even more restrictive than the currently condition. "if not dnsnames" looks like it would fit the bill better.

Also, better if you can provide a complete patch, including additional test(s) in Lib/test/test_ssl.py.
(see http://docs.python.org/devguide/runtests.html if you want information about running/writing tests)