Node.js – returned from Mongoose query that finds no matches

I'm a little confused reading the Mongoose documentation.

If I run a query in mongoose which matches no documents in the collection, what are the values of err and results in the callback function callback(err, results)? I just don't know what Mongoose considers an "error". As a mathematician, returning the empty set (i.e. results array empty) seems perfectly valid and shouldn't be an "error" – the query executed fine, there was just no matching documents. On the other hand, some may consider it an "error". From mongoose docs, either:

1. err = null, results = []
2. err = null, results = null
3. err = error document, results = null