I don't like the behaviour where if you just so happen to have 2 indices the behaviour changes.
I'd be tempted to raise a warning here stating that the behaviour has changed and in the warning give an example of how to use the new behaviour, but no matter what, an iterable is considered to be a fancy index.

Obviously this is a backwards incompatible change, so we will need an entry in the ```docs/api/api_changes.rst```.