Celery raises `ValueError: not enough values to unpack (expected 3, got 0)` when calling task

I'm using Celery in a Django project to schedule notification tasks. When I start the Celery worker and the task runs, I get the following error:

ValueError: not enough values to unpack (expected 3, got 0)

Full traceback:

File "celery\app\trace.py", line 640, in fast_trace_task
    tasks, accept, hostname = _loc
ValueError: not enough values to unpack (expected 3, got 0)

This happens when the following task is triggered:

# core/tasks.py

from celery import shared_task
from django.utils import timezone
from core.models import Notification

@shared_task
def send_scheduled_notifications():
    now = timezone.now()
    notifications = Notification.objects.filter(
        is_read=False,
        send_at__lte=now
    )
    for notification in notifications:
        print(f"Sending notification: {notification.title}")
        notification.is_read = True
        notification.save()

Celery starts successfully and logs the task as received, but then immediately crashes with the error above.

My environment:

  • Windows 11
  • Python 3.13
  • Celery 5.5.1
  • Redis running locally

I’ve confirmed the task runs fine when executed manually outside Celery.

"I came across this similar question, but my error occurs in a different version of Celery and the task seems structured correctly."

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