def _next_iter_line(self, row_num):
        """
        Wrapper around iterating through `self.data` (CSV source).

        When a CSV error is raised, we check for specific
        error messages that allow us to customize the
        error message displayed to the user.

        Parameters
        ----------
        row_num : The row number of the line being parsed.
        """

        try:
            return next(self.data)
        except csv.Error as e:
            if self.warn_bad_lines or self.error_bad_lines:
                msg = str(e)

                if 'NULL byte' in msg:
                    msg = ('NULL byte detected. This byte '
                           'cannot be processed in Python\'s '
                           'native csv library at the moment, '
                           'so please pass in engine=\'c\' instead')

                if self.skipfooter > 0:
                    reason = ('Error could possibly be due to '
                              'parsing errors in the skipped footer rows '
                              '(the skipfooter keyword is only applied '
                              'after Python\'s csv library has parsed '
                              'all rows).')
                    msg += '. ' + reason

                self._alert_malformed(msg, row_num)
            return None