Celery Errors in task in chord in chain not handled

The Fix

pip install celery==5.6.0

Based on closed celery/celery issue #6792 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -2234,7 +2234,8 @@ def run(self, header, body, partial_args, app=None, interval=None,
             body.options.update(options)
 
-        bodyres = body.freeze(task_id, root_id=root_id)
+        body_task_id = task_id or uuid()
+        bodyres = body.freeze(body_task_id, group_id=group_id, root_id=root_id)

repro.py

import celery
from celery.app.task import Task
from celery import shared_task

@shared_task(base=Task)
def echo_task(taskdata):
    return taskdata

@shared_task(base=Task)
def error_task(taskdata):
	1/0
	return taskdata

exec_list = [error_task.s(), echo_task.s()]
x = celery.chord(exec_list, echo_task.s())

y = celery.chain([x, echo_task.s()])

result = y(None).get()

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install celery==5.6.0\nWhen NOT to use: This fix is not applicable if the task ID is intentionally set to None.\n\n

Why This Fix Works in Production

Trigger: Exception raised outside body: TypeError('sequence item 1: expected a bytes-like object, NoneType found',):
Mechanism: The task ID could be None when using a chain as the body of a chord
Why the fix works: Fixes an issue where the task ID could be None when using a chain as the body of a chord, causing errors during error handling. (first fixed release: 5.6.0).

Why This Breaks in Prod

Shows up under Python 3.6 in real deployments (not just unit tests).
The task ID could be None when using a chain as the body of a chord
Surfaces as: Exception raised outside body: TypeError('sequence item 1: expected a bytes-like object, NoneType found',):

Proof / Evidence

GitHub issue: #6792
Fix PR: https://github.com/celery/celery/pull/9774
First fixed release: 5.6.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.19

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“> Are you saying it still reproduces? @DavraYoung can you check if it reproduces with the current main please? @Nusnus yes, it still reproduces with…”

@stuton · 2024-08-23 · source

“Yep I'm getting the same problem also. Trying to use a chord with a group of tasks with a call back and also a custom…”

@atomey · 2025-01-15 · source

“This should have solved by https://github.com/celery/celery/pull/9774”

@dfm88 · 2025-06-26 · source

“Hey @aditya051293 :wave:, Thank you for opening an issue”

@open-collective-bot · 2021-05-31 · source

Failure Signature (Search String)

Exception raised outside body: TypeError('sequence item 1: expected a bytes-like object, NoneType found',):

Error Message

Stack trace

error.txt

Error Message
-------------
Exception raised outside body: TypeError('sequence item 1: expected a bytes-like object, NoneType found',):
Traceback (most recent call last):
  File "/home/aditya/local/venv/lib/python3.6/site-packages/celery/backends/redis.py", line 519, in on_chord_part_return
    resl = [unpack(tup, decode) for tup in resl]
  File "/home/aditya/local/venv/lib/python3.6/site-packages/celery/backends/redis.py", line 519, in <listcomp>
    resl = [unpack(tup, decode) for tup in resl]
  File "/home/aditya/local/venv/lib/python3.6/site-packages/celery/backends/redis.py", line 425, in _unpack_chord_result
    raise ChordError(f'Dependency {tid} raised {retval!r}')
celery.exceptions.ChordError: Dependency b07a3b72-89c8-4c6b-9d7b-be24cda9af61 raised TaskException('ZeroDivisionError: division by zero',)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/aditya/local/venv/lib/python3.6/site-packages/celery/app/trace.py", line 518, in trace_task
    uuid, retval, task_request, publish_result,
  File "/home/aditya/local/venv/lib/python3.6/site-packages/celery/backends/base.py", line 163, in mark_as_done
    self.on_chord_part_return(request, state, result)
  File "/home/aditya/local/venv/lib/python3.6/site-packages/celery/backends/redis.py", line 538, in on_chord_part_return
    return self.chord_error_from_stack(callback, exc)
  File
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
[2022-02-11 10:44:23,017: ERROR/ForkPoolWorker-10] Task core.engine.tasks.rule.run_rule[a2123704-bbc4-47f1-8ca4-9d0734d57c80] raised unexpected: TypeError('sequence item 1: expected a bytes-like object, NoneType found')             Traceback (most recent call last):
  File "/root/.cache/pypoetry/virtualenvs/miniscanner--a4Dn15j-py3.8/lib/python3.8/site-packages/celery/app/trace.py", line 451, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/root/.cache/pypoetry/virtualenvs/miniscanner--a4Dn15j-py3.8/lib/python3.8/site-packages/celery/app/trace.py", line 734, in __protected_call__
    return self.run(*args, **kwargs)
  File "/data/sourcecode/miniscanner/core/engine/tasks/rule.py", line 54, in run_rule                                   scanner.verify()
  File "<string>", line 51, in verify
KeyError: 'sad'

During handling of the above exception, another exception occurred:
                                                                                                                    Traceback (most recent call last):
  File "/root/.cache/pypoetry/virtualenvs/miniscanner--a4Dn15j-py3.8/lib/python3.8/site-packages/celery/backends/redis.py", line 520, in on_chord_part_return                                                                               resl = [unpack(tup, decode) for tup in resl]
  File "/root/.cache/pypoetry/virtualenvs/miniscanner--a4Dn15j-py3.8/lib
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
[2024-08-19 09:20:22,043: ERROR/ForkPoolWorker-5] Task assistant.tasks.channels_generation[e5b492ec-ca00-40ec-911f-b5917205d009] raised unexpected: ValueError('task_id must not be empty. Got None instead.')
 Traceback (most recent call last):
   File "/usr/local/lib/python3.12/site-packages/celery/backends/redis.py", line 528, in on_chord_part_return
     resl = [unpack(tup, decode) for tup in resl]
             ^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.12/site-packages/celery/backends/redis.py", line 434, in _unpack_chord_result
     raise ChordError(f'Dependency {tid} raised {retval!r}')
 celery.exceptions.ChordError: Dependency 0163a4bc-da03-48f3-99eb-acabc9e0a0a8 raised AttributeError("'NoneType' object has no attribute 'group'")

 During handling of the above exception, another exception occurred:

 Traceback (most recent call last):
   File "/usr/local/lib/python3.12/site-packages/celery/app/trace.py", line 520, in trace_task
     task.backend.mark_as_done(
   File "/usr/local/lib/python3.12/site-packages/celery/backends/base.py", line 159, in mark_as_done
     self.on_chord_part_return(request, state, result)
   File "/usr/local/lib/python3.12/site-packages/celery/backends/redis.py", line 547, in on_chord_part_return
     return self.chord_error_from_stack(callback, exc)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   File "/usr/local/lib/python3.12/s
... (truncated) ...

Minimal Reproduction

repro.py

import celery
from celery.app.task import Task
from celery import shared_task

@shared_task(base=Task)
def echo_task(taskdata):
    return taskdata

@shared_task(base=Task)
def error_task(taskdata):
	1/0
	return taskdata

exec_list = [error_task.s(), echo_task.s()]
x = celery.chord(exec_list, echo_task.s())

y = celery.chain([x, echo_task.s()])

result = y(None).get()

Environment

Python: 3.6

What Broke

Errors occurred during error handling, leading to unexpected exceptions.

Why It Broke

The task ID could be None when using a chain as the body of a chord

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install celery==5.6.0

When NOT to use: This fix is not applicable if the task ID is intentionally set to None.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/celery/celery/pull/9774

First fixed release: 5.6.0

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

This fix is not applicable if the task ID is intentionally set to None.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
5.6.0	Fixed

Related Issues

No related fixes found.

Cluster: celery:other Celery hub Celery best practices All hubs All clusters

Related clusters: Configuration error Data consistency Duplicates

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Celery Errors in task in chord in chain not handled correctly (Fix)

The Fix

Why This Fix Works in Production

Why This Breaks in Prod

Proof / Evidence

Discussion

Failure Signature (Search String)

Error Message

Minimal Reproduction

Environment

What Broke

Why It Broke

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Get updates

When NOT to Use This Fix

Verify Fix

Did This Fix Work in Your Case?

Prevention

Version Compatibility Table

Related Issues

Sources