Python – celery tutorial: NotRegistered error

I'm trying to get celery's official tutorial work but kept getting this error:

D:\test>celery -A tasks worker –loglevel=info
————– celery@BLR122S v3.0.17 (Chiastic Slide)
—- **** —–
— * * * — [Configuration]
— * – **** — . broker: amqp://guest@localhost:5672//
– ** ———- . app: tasks:0x2a76850
– ** ———- . concurrency: 2 (processes)
– ** ———- . events: OFF (enable -E to monitor this worker)
– ** ———-
– * — * — [Queues]
— ******* —- . celery: exchange:celery(direct) binding:celery
— ***** —–
[Tasks]
. tasks.add
[2013-03-29 17:50:52,533: WARNING/MainProcess] celery@BLR122S ready.
[2013-03-29 17:50:52,568: INFO/MainProcess] consumer: Connected to amqp://guest@
127.0.0.1:5672//.
[2013-03-29 17:51:32,496: INFO/MainProcess] Got task from broker: tasks.add[8345
9233-ce54-40ed-a2a8-ee0d60768006]
[2013-03-29 17:51:32,562: ERROR/MainProcess] Task tasks.add[83459233-ce54-40ed-a
2a8-ee0d60768006] raised exception: Task of kind 'tasks.add' is not registered,
please make sure it's imported.
Traceback (most recent call last):

File "C:\Python27\lib\site-packages\billiard\pool.py", line 293, in worker

result = (True, func(*args, **kwds))
File "C:\Python27\lib\site-packages\celery\task\trace.py", line 320, in _fast_trace_task
return _tasks[task].__trace__(uuid, args, kwargs, request)[0]
File "C:\Python27\lib\site-packages\celery\app\registry.py", line 20, in __missing__
raise self.NotRegistered(key)
NotRegistered: 'tasks.add'

I installed celery==3.0.17 and rabbitMQ.
Then start celery by "D:\test>celery -A tasks worker --loglevel=info"
tasks.add seems to be in [Tasks], but calling by:

>>> from tasks import add
>>> add.delay(1,1)
# Out: AsyncResult: 83459233-ce54-40ed-a2a8-ee0d60768006

got the failure above. Does anyone have the same problem?

Edit:
Here is my tasks.py copying from tutorial.

from celery import Celery

celery = Celery('tasks', broker='amqp://guest@localhost//')

@celery.task
def add(x, y):
    return x + y