None is passed as an argument to ```field.clean```, which expects:

```def clean(self, value, model_instance):``` [Line 502](https://github.com/django/django/blob/master/django/db/models/fields/__init__.py#L502)

There isn't a model instance here, so I passed None to the function - otherwise it always fails due to not passing it an instance. I may have missed some underlying issue with this - please comment if so!