CPython assertion error `Py_REFCNT` when running compiled `mypy`

[johnyf]

Crash Report

Running mypy example.py results in a CPython assertion error.

Traceback

python: ./Include/internal/pycore_dict.h:288: _PyDict_NotifyEvent: Assertion `Py_REFCNT((PyObject*)mp) > 0' failed.
Aborted

Passing --show-traceback results in the same error, without additional information.

To Reproduce

Create a file example.py that contains:

def f(x):
    pass

Run:

mypy example.py

This error is only when using the compiled version of mypy. The pure Python version (installed with pip install --no-binary mypy mypy) works without errors.

Your Environment

• mypy 2.3.1 (compiled: yes) (installed from PyPI)
• no command-line flags
• no mypy.ini file
• Python 3.14.7+ (heads/3.14:1b1c61a94f0, Aug 8 2026, 07:47:02) [GCC 14.2.0] on linux
  (Python was compiled using ./configure ... --with-assertions)
• Linux Debian 13