Error adding a role in the Admin screen with an empty parameter list

Using Django 2.2.15 and just the django-prbac app installed, I tried to add a new role from the admin screen with an empty parameter list and received the following exception:

Internal Server Error: /admin/django_prbac/role/add/
Traceback (most recent call last):
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/contrib/admin/options.py", line 606, in wrapper
    return self.admin_site.admin_view(view)(*args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/utils/decorators.py", line 142, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/views/decorators/cache.py", line 44, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/contrib/admin/sites.py", line 223, in inner
    return view(request, *args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1645, in add_view
    return self.changeform_view(request, None, form_url, extra_context)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/utils/decorators.py", line 45, in _wrapper
    return bound_method(*args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/utils/decorators.py", line 142, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1529, in changeform_view
    return self._changeform_view(request, object_id, form_url, extra_context)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1572, in _changeform_view
    self.save_model(request, new_object, form, not add)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/contrib/admin/options.py", line 1088, in save_model
    obj.save()
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django_prbac/models.py", line 30, in save
    super(ValidatingModel, self).save(force_insert, force_update, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/base.py", line 741, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/base.py", line 779, in save_base
    force_update, using, update_fields,
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/base.py", line 870, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/base.py", line 908, in _do_insert
    using=using, raw=raw)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/query.py", line 1186, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1374, in execute_sql
    for sql, params in self.as_sql():
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1318, in as_sql
    for obj in self.query.objs
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1318, in <listcomp>
    for obj in self.query.objs
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1317, in <listcomp>
    [self.prepare_value(field, self.pre_save_val(field, obj)) for field in fields]
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1258, in prepare_value
    value = field.get_db_prep_save(value, connection=self.connection)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/fields/__init__.py", line 793, in get_db_prep_save
    return self.get_db_prep_value(value, connection=connection, prepared=False)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django/db/models/fields/__init__.py", line 788, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/Users/sgraham/.local/share/virtualenvs/temp2-ec52t3-K/lib/python3.7/site-packages/django_prbac/fields.py", line 99, in get_prep_value
    raise ValueError('Invalid value %r for StringSetField' % value)
ValueError: Invalid value [] for StringSetField```
dimagi / django-prbac

Error adding a role in the Admin screen with an empty parameter list #63
