> 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. 

but an alias to a single index is allowed? If "yes", then it needs to be an exception, as it's not under the code control what input the user provides (the user can provide an alias that points to multiple indices... )

> 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).

fair point... if it's called all over the place, then perhaps the exception should be thrown here (with somewhat of a generic yet descriptive & user friendly message)