anchor.diff:        }
   
         if (connection == null) {
  -        connection = _masterConnection;
  +        if (_masterConnection != null) {
  +          connection = _masterConnection;
  +        }
  +        else {
  +          throw new MongoDartError('No master connection');
  +        }
         }
   
         return connection.query(queryMessage);
  
positive.code:     }
  
      if (connection == null) {
        connection = _masterConnection;
      }
  
      if (writeConcern == null) {
  
negative.code:   Future executeDbCommand(MongoMessage message,
        {_Connection connection}) async {
      if (connection == null) {
        if (_masterConnection != null) {
          connection = _masterConnection;
        }
        else {
          throw new MongoDartError('No master connection');
        }
      }
  
      Completer<Map> result = new Completer();
  