This line:

https://github.com/mongodb/node-mongodb-native/blob/3.1/lib/operations/collection_ops.js#L237

will throw an error if the query to countDocuments does not match:

db.collection('somecollection').countDocuments({foo: 'blah'})

Uncaught TypeError: Cannot read property 'n' of undefined

`docs` is an empty array if the query does not match.