We currently use

{'$group': {'_id': null, 'n': {'$sum': 1} } }

as an aggregation stage for the countDocuments. The _id can be any constant value, like 1 or 0. Since it can be any constant value, we should change the group stage to:

{'$group': {'_id': 1, 'n': {'$sum': 1} } }

This helps us with test runners and languages that consider null to be an indication that a key is not present.