How to use the croniter.CroniterNotAlphaError function in croniter

def validate_cron(value):
    if value == '':
        return

    now = timezone.now()
    try:
        croniter(value, now)
    except (CroniterBadCronError, CroniterBadDateError, CroniterNotAlphaError):
        raise ValidationError('Invalid value cron')

for dag in list(m.__dict__.values()):
                if isinstance(dag, DAG):
                    if not dag.full_filepath:
                        dag.full_filepath = filepath
                        if dag.fileloc != filepath and not is_zipfile:
                            dag.fileloc = filepath
                    try:
                        dag.is_subdag = False
                        self.bag_dag(dag, parent_dag=dag, root_dag=dag)
                        if isinstance(dag._schedule_interval, str):
                            croniter(dag._schedule_interval)
                        found_dags.append(dag)
                        found_dags += dag.subdags
                    except (CroniterBadCronError,
                            CroniterBadDateError,
                            CroniterNotAlphaError) as cron_e:
                        self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                        self.import_errors[dag.full_filepath] = \
                            "Invalid Cron expression: " + str(cron_e)
                        self.file_last_changed[dag.full_filepath] = \
                            file_last_changed_on_disk
                    except AirflowDagCycleException as cycle_exception:
                        self.log.exception("Failed to bag_dag: %s", dag.full_filepath)
                        self.import_errors[dag.full_filepath] = str(cycle_exception)
                        self.file_last_changed[dag.full_filepath] = \
                            file_last_changed_on_disk

        self.file_last_changed[filepath] = file_last_changed_on_disk
        return found_dags