importer stopped working

web.models.study_subject.StudySubject.type.RelatedObjectDoesNotExist: StudySubject has no type.
Traceback (most recent call last):
  File "/var/www/scheduling-system/smash/web/importer/importer.py", line 43, in execute
    self.import_study_subject(study_subject)
  File "/var/www/scheduling-system/smash/web/importer/importer.py", line 66, in import_study_subject
    new_value = self.get_new_value(old_value, getattr(study_subject, field.name))
  File "/var/www/scheduling-system/env/lib/python3.6/site-packages/django/db/models/fields/related_descriptors.py", line 198, in __get__
    "%s has no %s." % (self.field.model.__name__, self.field.name)
web.models.study_subject.StudySubject.type.RelatedObjectDoesNotExist: StudySubject has no type.