TypeError: issubclass() arg 1 must be a class

[priteau]

What version of myst-parser are you using?

v5.0.0

What version dependencies are you using?

Sphinx 9.1.0
docutils 0.21.2

What operating system are you using?

Linux

Describe the Bug

We are seeing the following the issue when building our documentation with myst-parser 5.0.0:

Traceback (most recent call last):
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/cmd/build.py", line 432, in build_main
    app.build(args.force_all, args.filenames)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/application.py", line 442, in build
    self.builder.build_update()
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 381, in build_update
    self.build(
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 460, in build
    self.write(docnames, updated_docnames, method)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 748, in write
    self.write_documents(docnames)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 762, in write_documents
    self._write_serial(sorted_docnames)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 777, in _write_serial
    _write_docname(docname, env=self.env, builder=self, tags=self.tags)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/builders/__init__.py", line 887, in _write_docname
    doctree = env.get_and_resolve_doctree(docname, builder=builder, tags=tags)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/environment/__init__.py", line 699, in get_and_resolve_doctree
    self.apply_post_transforms(doctree, docname)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/environment/__init__.py", line 771, in apply_post_transforms
    transformer.apply_transforms()
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/transforms/__init__.py", line 95, in apply_transforms
    super().apply_transforms()
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/docutils/transforms/__init__.py", line 182, in apply_transforms
    transform.apply(**kwargs)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/transforms/post_transforms/__init__.py", line 46, in apply
    self.run(**kwargs)
  File "/home/ubuntu/stackhpc-kayobe-config/.tox/docs/lib/python3.12/site-packages/sphinx/transforms/post_transforms/__init__.py", line 352, in run
    if issubclass(translator, SphinxTranslator):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: issubclass() arg 1 must be a class

We are not using MyST-Parser directly, but only through sphinx-substitution-extensions.

Expected Behavior

Documentation builds successfully.

To Reproduce

Build docs at https://github.com/stackhpc/stackhpc-kayobe-config/tree/stackhpc/2025.1/doc/source