django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet

settings

INSTALLED_APPS = [
    "django.contrib.admin",
    "django.contrib.auth",
    "django.contrib.contenttypes",
    "django.contrib.sessions",
    "django.contrib.messages",
    "django.contrib.staticfiles",
    "data_access",
    "api.vehicles.apps.VehiclesApiConfig",
]

api.vehicles.apps

from django.apps import AppConfig
from django.db.models.signals import post_save, post_delete

from api.vehicles.signals import vehicle_save_signal_receiver, vehicle_deleted_signal_receiver
from data_access.models.vehicle import Vehicle


class VehiclesApiConfig(AppConfig):
    name = "api.vehicles"

    def ready(self):
        post_save.connect(receiver=vehicle_save_signal_receiver, sender=Vehicle)
        post_delete.connect(receiver=vehicle_deleted_signal_receiver, sender=Vehicle)

api.vehicles.signals

def vehicle_save_signal_receiver(sender, **kwargs):
    print("vehicle_save_signal_receiver")


def vehicle_deleted_signal_receiver(sender, **kwargs):
    print("vehicle_deleted_signal_receiver")

ошибка django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet. на импорте Vehicle в api.vehicles.apps, но data_access в INSTALLED_APPS перед api.vehicles

почему такое может быть ?