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