Mail::SPF::Query not recognising secondary MX using IPv6

I use spf-milter 1.41 with sendmail. I use the -mx parameter as my
ISP provides a secondary MX. The secondary MX system now sends mail
to me using IPv6. Previously when (the same) secondary MX sent mail
using IPv4, my system generated an SPF pass with the comment that it
had arrived from a secondary MX. Now that it is using IPv6, I get no
Received-SPF header added.