both good points... the reason why it's an assert is that it should never happen, as we pass in a single index and we say we don't resolve aliases to multiple indices. Can make it an exception but then it would seem that we handle the case while this must not happen unless there are bugs.

As for removing the method entirely. I like the idea, but then we'd end up with these same two lines all over the place...not sure it's great, I thought it's better to share those lines (length check and array to single string conversion).