BUG: df.join(df2, how='right') TypeError
Issue #11519

Somehow right joins had been forgotten in a previous bugfix. There were tests already written that should have seen the problem, but they had been commented out because the expected results were wrong, because of a subtlety in the way non-unique index are handled.

Thanks for the labels "difficulty novice" and "effort low": I'm using pandas every day, and I'm glad I could contribute that easily.
