KeyError Received unregistered task of type '' on celery while task is registered

I'm a bit new in celery configs.

I have a task named myapp.tasks.my_task for example.

I can see myapp.tasks.my_task in registered tasks of celery when I use celery inspect registered. doesn't it mean that the task is successfully registered? why it raises the following error for it:

KeyError celery.worker.consumer.consumer in on_task_received

Received unregistered task of type 'my_app.tasks.my_task'.
The message has been ignored and discarded.

Did you remember to import the module containing this task?
Or maybe you're using relative imports?

Please see
http://docs.celeryq.org/en/latest/internals/protocol.html
for more information.

The full contents of the message body was:
'[[], {}, {"callbacks": null, "errbacks": null, "chain": null, "chord": null}]' (77b)

there are also other tasks in my_app.tasks and they work correctly but only this task does not work and gets KeyError:

@shared_task(queue='celery')
def other_task():
   """ WORKS """
   ...

@shared_task(queue='celery')
def my_task():
   """ DOES NOT WORK """
   ...

Back in the day, when I faced the problem a senior solved it for me in a mushroom management way unfortunately (see more about this anti-pattern here). I came back to this problem recently to figure out the solution in our own project domain.

As Niel pointed in his/her solution, we were using celery_app.autodiscover_tasks() in our project and in that case we should import my_task in __init__.py of tasks package like bellow.

from .some_tasks_file import my_task

Also, we used celery beat and the task defined inside app.conf.beat_schedule must have exact path to the function like bellow (even though the function is imported in __init__.py of the tasks package).

app.conf.beat_schedule = {
    'MY_TASK': {
        'task': 'myapp.tasks.some_tasks_file.my_task',
        'schedule': 60,  # every minute
    },
}

Hope this would help people with the same celery configuration and problem.