TypeError from serve.serve logging

[gsnedders]

Somewhat mysteriously, in https://github.com/web-platform-tests/wpt/pull/49970/checks?check_run_id=35434087747 (https://dev.azure.com/web-platform-tests/wpt/_build/results?buildId=131812&view=ms.vss-test-web.build-test-results-tab&runId=1192631&resultId=100566&paneView=debug), Azure Pipelines apparently is getting:

pytest.PytestUnhandledThreadExceptionWarning: Exception in thread Thread-1 (target)

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/threading.py", line 1045, in _bootstrap_inner
    self.run()
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/threading.py", line 982, in run
    self._target(*self._args, **self._kwargs)
  File "/Users/runner/work/1/s/tools/serve/test_functional.py", line 68, in target
    serve.run(config_path=tempfile_name)
  File "/Users/runner/work/1/s/tools/serve/serve.py", line 1480, in run
    logger.warning('Status of subprocess "%s": failed. Exit with non-zero status: %d',
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 1501, in warning
    self._log(WARNING, msg, args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 1634, in _log
    self.handle(record)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 1644, in handle
    self.callHandlers(record)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 1706, in callHandlers
    hdlr.handle(record)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 978, in handle
    self.emit(record)
  File "/Users/runner/work/1/s/tools/.tox/py311/lib/python3.11/site-packages/_pytest/logging.py", line 386, in emit
    super().emit(record)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 1118, in emit
    self.handleError(record)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 1110, in emit
    msg = self.format(record)
          ^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 953, in format
    return fmt.format(record)
           ^^^^^^^^^^^^^^^^^^
  File "/Users/runner/work/1/s/tools/.tox/py311/lib/python3.11/site-packages/_pytest/logging.py", line 139, in format
    return super().format(record)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 687, in format
    record.message = record.getMessage()
                     ^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/logging/__init__.py", line 377, in getMessage
    msg = msg % self.args
          ~~~~^~~~~~~~~~~
TypeError: %d format: a real number is required, not NoneType

(This doesn't appear in any of the run logs, which is strange, but I can well believe this is a genuine intermittent failure.)