Python binding method 'declineOffer(offerid, filters=None)' raises exception when 'filters=None' is assigned explicitly

def launchTasks(self, offerIds, tasks, filters=None):  # The method's signature.
   ...

declineOffer(offerId)  # OK to call it this way
declineOffer(offerId, filters=None)  # Error when calling it this way

if (obj == Py_None) {
    std::cerr << "None object given where protobuf expected" << std::endl;
    return false;
 }

  if (filtersObj != NULL) {
    if (!readPythonProtobuf(filtersObj, &filters)) {
      PyErr_Format(PyExc_Exception,
                   "Could not deserialize Python Filters");
      return NULL;
    }
  }