'Regression test for #13071: NullBooleanField should not throw
a validation error when given a value of None.'

def test_nullbooleanfield_blank(self):

    nullboolean = NullBooleanModel(nbfield=None)
    try:
        nullboolean.full_clean()
    except ValidationError as e:
        self.fail((u'NullBooleanField    failed    validation    with    value    of    None:    %s' % e.messages))
