django-filer: FilerFile model not being saved; NoneType error raised instead. Why? -

hi i'm trying test mock django-filer save() trigger post_save signal.

models.py

import filer.fields.file import filerfilefield  class deliveryinvoice(timestampedmodel):     invoice_excel = filerfilefield(null=true, blank=true) 

tests.py

from filer.models.filemodels import file filerfile django.core.files import file  def test_if_delivery_invoice_number_updated_on_file_save_through_admin(self):     patch("orders.utils.apply_invoice_number_to_orders") signal_mock_handler:         post_save.connect(signal_mock_handler, sender=deliveryinvoice)          filename = 'test_invoice_excel'         filepath = 'orders/fixtures/delivery_invoices.xlsx'          open(filepath, 'rb') f:             file_obj = file(f, name=filename) error ->    invoice_excel = filerfile.objects.create(owner=self.user, file=file_obj, original_filename=filename)             instance = deliveryinvoice(invoice_excel=invoice_excel)             instance.save()      self.asserttrue(signal_mock_handler.called) 

error message

.... file "/users/mhjeon/.pyenv/versions/3.6.0/envs/modernlab/lib/python3.6/site-packages/boto/auth.py", line 1070, in _wrapper if test in self.host:  typeerror: argument of type 'nonetype' not iterable 

the code used work few days ago, after code refactoring believe not related orders models, fail calle django-filer's filermodel.save() method. wrong?/