AppRegistryNotReady("Apps aren't loaded yet.")

[james-eng]

Just installed and set up. Everything went smoothly, including migration, until adding

from captcha.fields import CaptchaField

Using:

django-simple-captcha==0.5.9
Django==2.0.6
pilkit==2.0
Pillow==5.2.0
six==1.11.0
django-ranged-response==0.2.0

Trace:

Unhandled exception in thread started by <function check_errors.<locals>.wrapper at 0x7f17ae3a68c8>
Traceback (most recent call last):
  File "<project_root>/venv/lib/python3.5/site-packages/django/utils/autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "<project_root>/venv/lib/python3.5/site-packages/django/core/management/commands/runserver.py", line 112, in inner_run
    autoreload.raise_last_exception()
  File "<project_root>/venv/lib/python3.5/site-packages/django/utils/autoreload.py", line 248, in raise_last_exception
    raise _exception[1]
  File "<project_root>/venv/lib/python3.5/site-packages/django/core/management/__init__.py", line 327, in execute
    autoreload.check_errors(django.setup)()
  File "<project_root>/venv/lib/python3.5/site-packages/django/utils/autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "<project_root>/venv/lib/python3.5/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "<project_root>/venv/lib/python3.5/site-packages/django/apps/registry.py", line 89, in populate
    app_config = AppConfig.create(entry)
  File "<project_root>/venv/lib/python3.5/site-packages/django/apps/config.py", line 90, in create
    module = import_module(entry)
  File "/usr/lib/python3.5/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 986, in _gcd_import
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 958, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 673, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 665, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "<project_root>/letters/forms.py", line 3, in <module>
    from captcha.fields import CaptchaField
  File "<project_root>/venv/lib/python3.5/site-packages/captcha/fields.py", line 4, in <module>
    from captcha.models import CaptchaStore
  File "<project_root>/venv/lib/python3.5/site-packages/captcha/models.py", line 25, in <module>
    class CaptchaStore(models.Model):
  File "<project_root>/venv/lib/python3.5/site-packages/django/db/models/base.py", line 100, in __new__
    app_config = apps.get_containing_app_config(module)
  File "<project_root>/venv/lib/python3.5/site-packages/django/apps/registry.py", line 244, in get_containing_app_config
    self.check_apps_ready()
  File "<project_root>/venv/lib/python3.5/site-packages/django/apps/registry.py", line 127, in check_apps_ready
    raise AppRegistryNotReady("Apps aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.