KeyError: 'carry_over_week' if preference is disabled

Internal Server Error: /app/alsijil/lesson/2022/23/5278
Traceback (most recent call last):
  File "/usr/local/src/venv/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/local/src/venv/lib/python3.9/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/src/venv/lib/python3.9/site-packages/rules/contrib/views.py", line 231, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/usr/local/src/AlekSIS/apps/official/AlekSIS-App-Alsijil/aleksis/apps/alsijil/views.py", line 264, in register_object
    lesson_documentation_form.save()
  File "/usr/local/src/AlekSIS/apps/official/AlekSIS-App-Alsijil/aleksis/apps/alsijil/forms.py", line 61, in save
    self.cleaned_data["carry_over_week"]
KeyError: 'carry_over_week'