code: def fillna(self, series, addition=0):
          
          if series.dtype == numpy.object:
              return series
  
          return series.fillna(self.missing_value + addition).astype(self.dtype)
docs: Fills with encoder specific default values.
  
          :param data: examined to determine defaults
          :param addition: uniquely identify this set of fillnas if necessary
          :return: filled data