I got the following error (Python 3.7.8):

Traceback (most recent call last):
  File "/home/equaeghe/.local/bin/html2alternative.py", line 68, in <module>
    replaceable.add_alternative(plain)
  File "/usr/lib/python3.7/email/message.py", line 1153, in add_alternative
    self._add_multipart('alternative', *args, **kw)
  File "/usr/lib/python3.7/email/message.py", line 1144, in _add_multipart
    part.set_content(*args, **kw)
  File "/usr/lib/python3.7/email/message.py", line 1171, in set_content
    super().set_content(*args, **kw)
  File "/usr/lib/python3.7/email/message.py", line 1101, in set_content
    content_manager.set_content(self, *args, **kw)
  File "/usr/lib/python3.7/email/contentmanager.py", line 37, in set_content
    handler(msg, obj, *args, **kw)
  File "/usr/lib/python3.7/email/contentmanager.py", line 185, in set_text_content
    cte, payload = _encode_text(string, charset, cte, msg.policy)
  File "/usr/lib/python3.7/email/contentmanager.py", line 154, in _encode_text
    max(len(x) for x in lines) <= policy.max_line_length):
TypeError: '<=' not supported between instances of 'int' and 'NoneType'

This is triggered because it is incorrectly assumed that policy.max_line_length cannot be None. This issue is still present in current master:

https://github.com/python/cpython/blob/c3dd7e45cc5d36bbe2295c2840faabb5c75d83e4/Lib/email/contentmanager.py#L149

Are you able to post a script that reproduces the issue?

The script that triggered the issue can be found at

https://github.com/equaeghe/mailfilters/blob/master/html2alternative.py

You'll have to remove ", cte='8bit'" on line 68 to expose the bug (that was added as a workaround for this bug).

I was consistently able to trigger the bug when applying it to a mail with a multipart/alternative with text and html parts already present. It should replace that html part by a (nested) multipart/alternative part, but it crashes unless you keep the workaround.

Seems to be a duplicate of #34800