Instead of omitting the writeConcern when no fields are set, PyMongo adds an empty writeConcern. This only happens with Python 2. The fix is to define the __nonzero__ method on the WriteConcern class. __nonzero__ was renamed to __bool__ in Python 3.

Note: I don't think there is any difference on the server between calling a command with an empty writeConcern document vs omitting the writeConcern. This will just save some bytes on the network and make pymongo adhere to the read-write-concern spec.