Requests Preparing a request body without headers throws a

The Fix

pip install requests==2.27.0

Based on closed psf/requests issue #5504 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -273,7 +273,9 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
     containing the exact bytes that will be sent to the server.
 
-    Generated from either a :class:`Request <Request>` object or manually.
+    Instances are generated from a :class:`Request <Request>` object, and
+    should not be instantiated manually; doing so may produce undesirable

repro.py

import requests

if __name__ == "__main__":
    req = requests.PreparedRequest()
    req.prepare_body(data="hello world!", files=None)
    print(req.body)

verify

Re-run: python3 main.py

fix.md

Option A — Upgrade to fixed release\npip install requests==2.27.0\nWhen NOT to use: This fix is not applicable if manual instantiation of PreparedRequest is intended.\n\n

Why This Fix Works in Production

Trigger: $ python3 main.py
Mechanism: The method prepare_body fails when headers are None, leading to a TypeError
Why the fix works: Clarifies that PreparedRequests should not be manually created to avoid undesirable effects. (first fixed release: 2.27.0).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.6 in real deployments (not just unit tests).
The method prepare_body fails when headers are None, leading to a TypeError
Surfaces as: $ python3 main.py

Proof / Evidence

GitHub issue: #5504
Fix PR: https://github.com/psf/requests/pull/5505
First fixed release: 2.27.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.48

Discussion

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

Jump to Sources Open on GitHub

“Right, you're expected to create a Request and then call prepare on that to get your own PreparedRequest”

@sigmavirus24 · 2020-06-18 · source

“> You really shouldn't be creating PreparedRequests Gotcha. The API docs for it say manual creation is allowed, so if that's not the case, the…”

@ollien · 2020-06-18 · source

“That's a fair point. I'm sure Nate or Seth would merge a PR if you removed that language”

@sigmavirus24 · 2020-06-19 · source

“Yep, I think that's completely reasonable. Partial initialization has a number of failure points so we shouldn't be recommending it.”

@nateprewitt · 2020-06-19 · source

Failure Signature (Search String)

$ python3 main.py

Error Message

Stack trace

error.txt

Error Message
-------------
$ python3 main.py
Traceback (most recent call last):
  File "main.py", line 5, in <module>
    req.prepare_body(data="hello world!", files=None)
  File "/Users/nkrichevsky/.local/lib/python3.6/site-packages/requests/models.py", line 514, in prepare_body
    self.prepare_content_length(body)
  File "/Users/nkrichevsky/.local/lib/python3.6/site-packages/requests/models.py", line 529, in prepare_content_length
    self.headers['Content-Length'] = builtin_str(length)
TypeError: 'NoneType' object does not support item assignment

Minimal Reproduction

repro.py

import requests

if __name__ == "__main__":
    req = requests.PreparedRequest()
    req.prepare_body(data="hello world!", files=None)
    print(req.body)

Environment

Python: 3.6

What Broke

Attempting to prepare a request body without headers results in a TypeError and prevents request execution.

Why It Broke

The method prepare_body fails when headers are None, leading to a TypeError

Fix Options (Details)

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

pip install requests==2.27.0

When NOT to use: This fix is not applicable if manual instantiation of PreparedRequest is intended.

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

Fix reference: https://github.com/psf/requests/pull/5505

First fixed release: 2.27.0

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

When NOT to Use This Fix

This fix is not applicable if manual instantiation of PreparedRequest is intended.

Verify Fix

verify

Re-run: python3 main.py

Did This Fix Work in Your Case?

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

Prevention

Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
2.27.0	Fixed

Related Issues

No related fixes found.

Cluster: requests:configuration-error Requests hub Requests best practices All hubs All clusters

Related clusters: Data consistency SSL handshake Other

Sources

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

Requests Preparing a request body without headers throws a TypeError (Fix)