Django test parallel AppRegistryNotReady

I am trying to understand how I can run django tests in parallel.

I have django app with that structure:

gbook
    order
        ...
        tests
            __init__.py
            test_a1.py
            test_b1.py
            utils.py

test_a1.py and test_b1.py contains same code:

import time
from order import models
from .utils import BackendTestCase


class ATestCase(BackendTestCase):
    def test_a(self):
        time.sleep(1)
        a = models.City.objects.count()
        self.assertEqual(a, a)

class BTestCase(BackendTestCase):
    def test_b(self):
        time.sleep(1)
        a = models.City.objects.count()
        self.assertEqual(a, a)

utils.py is:

from django.test import TestCase, Client
from order import models
from django.conf import settings

from order.utils import to_hash


class BackendTestCase(TestCase):
    fixtures = ['City.json', 'Agency.json']

    def setUp(self):
        self.client = Client()
        self.lang_codes = (i[0] for i in settings.LANGUAGES)
        ...

settings_test.py:

from .settings import *

DEBUG = False

TEMPLATE_DEBUG = False

STATICFILES_STORAGE = 'django.contrib.staticfiles.storage.StaticFilesStorage'

PASSWORD_HASHERS = ['django.contrib.auth.hashers.MD5PasswordHasher',]  # faster

DATABASES['default'] = {
    'ENGINE': 'django.db.backends.sqlite3',
}

When I run test in single process, all goes well (about 4 sec):

python.exe manage.py test order --settings=gbook.settings_test

Then I trying to run tests in parallel:

python.exe manage.py test order --settings=gbook.settings_test --parallel=2

I get this trace:

Creating test database for alias 'default'...

Cloning test database for alias 'default'...
Cloning test database for alias 'default'...
System check identified no issues (0 silenced).
Process SpawnPoolWorker-2:
Process SpawnPoolWorker-1:
Traceback (most recent call last):
Traceback (most recent call last):
  File "C:\python\Python36-32\lib\multiprocessing\process.py", line 258, in _bootstrap
    self.run()
  File "C:\python\Python36-32\lib\multiprocessing\process.py", line 258, in _bootstrap
    self.run()
  File "C:\python\Python36-32\lib\multiprocessing\process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "C:\python\Python36-32\lib\multiprocessing\process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "C:\python\Python36-32\lib\multiprocessing\pool.py", line 108, in worker
    task = get()
  File "C:\python\Python36-32\lib\multiprocessing\pool.py", line 108, in worker
    task = get()
  File "C:\python\Python36-32\lib\multiprocessing\queues.py", line 337, in get
    return _ForkingPickler.loads(res)
  File "C:\python\Python36-32\lib\multiprocessing\queues.py", line 337, in get
    return _ForkingPickler.loads(res)
  File "C:\kvk\develop\Python\gbook\order\tests\test_a1.py", line 2, in <module>
    from order import models
  File "C:\kvk\develop\Python\gbook\order\tests\test_a1.py", line 2, in <module>
    from order import models
  File "C:\kvk\develop\Python\gbook\order\models.py", line 79, in <module>
    class Agency(models.Model):
  File "C:\kvk\develop\Python\gbook\order\models.py", line 79, in <module>
    class Agency(models.Model):
  File "C:\python\venv\gbook\lib\site-packages\django\db\models\base.py", line 110, in __new__
    app_config = apps.get_containing_app_config(module)
  File "C:\python\venv\gbook\lib\site-packages\django\db\models\base.py", line 110, in __new__
    app_config = apps.get_containing_app_config(module)
  File "C:\python\venv\gbook\lib\site-packages\django\apps\registry.py", line 247, in get_containing_app_config
    self.check_apps_ready()
  File "C:\python\venv\gbook\lib\site-packages\django\apps\registry.py", line 247, in get_containing_app_config
    self.check_apps_ready()
  File "C:\python\venv\gbook\lib\site-packages\django\apps\registry.py", line 125, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
  File "C:\python\venv\gbook\lib\site-packages\django\apps\registry.py", line 125, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

multiprocessing.cpu_count() = 4

Django version 1.11.10

Python 3.6.5

Docs says: "--parallel" Runs tests in separate parallel processes. Each process gets its own database. And I do not need to change my code for use it.

Please, help me to understand, what am i doing wrong.