Schedules with repetitions cause an errorneous playout endpoint

Schedules / Timeslots with repetition schemes make the API endpoint /api/v1/playout dysfunctional (reported by Renatn).

Internal Server Error: /api/v1/playout
Traceback (most recent call last):
  File "/home/laura/venv-aura-python/lib/python3.9/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/laura/venv-aura-python/lib/python3.9/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/laura/venv-aura-python/lib/python3.9/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/laura/steering/program/views.py", line 97, in json_playout
    is_repetition = ' ' + _('REP') if ts.schedule.is_repetition is True else ''
TypeError: can only concatenate str (not "__proxy__") to str
ERROR:django.request:Internal Server Error: /api/v1/playout

renatn_steering-log-15-12-2021-2