Unknown response / unknown advise for query advisor

Summary: Certain query when fed into query advisor returns "Unknown response from server"

Repro:

1. Install Couchbase Server enterprise
2. Create bucket with primary key (I named mine survey_results_public)
3. Enable developer preview mode (so that index advisor is enabled)
4. Run the query (see attached)

Actual:

Message "Unknown response from server" appears in the UI. See screenshot and see JavaScript response.

Expected:

This depends on what's happening behind the scenes.

Is the advisor unable to make a recommendation? There isn't any "reason" or additional information in the response from the server. It would be nice if there was at least a short reason given why no indexes could be recommended.

Another possibility is that this is a bug in the advisor.

and/or

This might be mitigated in the UI with a more descriptive error message. But it seems like the UI is really at the mercy of the backend here, so I'm not sure what that message could be. At the least, something like "Unable to recommend an index" is slightly better than "Unknown response from server".