I need a name for a logical fallacy [View all]

The fallacy is because Person <X> agrees with Person <Y> on Subject <A>, they must also agree on Subject <B>.

I came across this one in a discussion with a libertarian, when I told him that the first socialized medicine program was instituted by Otto von Bismarck, in part because he agreed with the Socialists that socialized medicine would be good for Germany. The libertarian then said that Bismarck must have been a socialist.