Ugh, it really took much more time than I expected... :smile: 

Anyway, I finally took a look at this attribute in Django, and the way it's created is a bit convoluted. In any case, Django expects a `form.base_fields` to be a `OrderedDict`, but if it's empty it can be a dict as well. It is not necessary to set it in the class declaration, it could go into `__init__` or even `__new__` methods.