pytest "INTERNALERROR>" - pytest Itself Crashed in CI

An INTERNALERROR> means pytest itself (or one of its plugins) raised an unhandled exception - during collection, hook execution, or reporting - rather than a test failing. The traceback points inside pytest or a plugin.

What this error means

The run ends with lines prefixed INTERNALERROR> and a traceback through _pytest/... or a plugin package, not your test code. No normal pass/fail summary follows because pytest crashed.

pytest output

INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File ".../_pytest/main.py", line 269, in wrap_session
INTERNALERROR>   File ".../pytest_randomly/__init__.py", line 200, in ...
INTERNALERROR> TypeError: object of type 'NoneType' has no len()

Common causes

An incompatible plugin version

A pytest plugin out of step with the pytest version (or another plugin) raises inside a hook, crashing the whole session.

A bad conftest or hook implementation

A custom hook in conftest.py that returns the wrong type or raises during collection surfaces as an INTERNALERROR rather than a test failure.

How to fix it

Read the traceback to the offending package

Find the deepest INTERNALERROR> frame - it names the plugin or conftest at fault.
Pin or upgrade that plugin to a version compatible with your pytest.
If it is your conftest hook, fix its return type/exception.

Bisect plugins to isolate the culprit

Disable plugins to find which one crashes the session.

Terminal

pytest -p no:randomly        # disable a suspect plugin
pytest -p no:cacheprovider   # or another
# pin compatible versions once identified
pip install "pytest==8.2.*" "pytest-randomly==3.15.*"

How to prevent it

Pin pytest and its plugins to a known-compatible set.
Upgrade pytest and plugins together, not piecemeal.
Keep custom hooks small and well-typed; test conftest changes locally.