Error from new-tsp-backup-job when not passing rsync args

sudo /usr/lib/chem-zfs-backup-server/new-tsp-backup-job --name OS_backups --hostname rocky9builder.ch.privaate.cam.ac.uk --create-zfs-property 'quota=250G' --src / --destpath zfs-rsnap
Traceback (most recent call last):
  File "/usr/lib/chem-zfs-backup-server/new-tsp-backup-job", line 124, in <module>
    main()
  File "/usr/lib/chem-zfs-backup-server/new-tsp-backup-job", line 120, in main
    append_backup_job(config_file, args)
  File "/usr/lib/chem-zfs-backup-server/new-tsp-backup-job", line 102, in append_backup_job
    f.write(template.render(args=args))
  File "/usr/lib/python3/dist-packages/jinja2/environment.py", line 1291, in render
    self.environment.handle_exception()
  File "/usr/lib/python3/dist-packages/jinja2/environment.py", line 925, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/usr/lib/chem-zfs-backup-server/templates/config.yml", line 11, in top-level template code
    {%- if args.rsync_args | length %}
TypeError: object of type 'NoneType' has no len()

Either there should be a default for rsync_args or it should be compulsory.