See test_read_prefs_mongos_secondary(). Given an empty query and a read preference, mongoc_collection_find() can send a document like this to mongos:

{'$readPreference': {'mode': 'secondary'}}

Mismatches the Server Selection Spec:

When any $ modifier is used, including the $readPreference modifier, the query MUST be provided using the $query modifier

The driver should send:

{'$query': {}, '$readPreference': {'mode': 'secondary'}}

https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst