def METHOD_NAME(objecttype):
    """Test the get_fields method for fields of the subtype.
    Test whether all returned fields have 'notnull' = False.
    Fields of a subtype cannot be mandatory since the subtype itself is not
    compulsory.
    """
    allfields = objecttype.get_field_names()
    ownfields = objecttype.get_field_names(include_subtypes=False)
    subfields = [f for f in allfields if f not in ownfields]
    fields = objecttype.get_fields(source=('xml',))
    for field in fields.values():
        if field['name'] in subfields:
            assert field['notnull'] is False