LookupError: unknown encoding: unknown-8bit

GNU Mailman 3.2.0a1.

This may be related to #203 (closed).

Jan 16 22:31:30 2018 (37) Uncaught runner exception: unknown encoding: unknown-8bit
Jan 16 22:31:30 2018 (37) Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/mailman/core/runner.py", line 173, in _one_iteration
    self._process_one_file(msg, msgdata)
  File "/usr/local/lib/python3.6/site-packages/mailman/core/runner.py", line 266, in _process_one_file
    keepqueued = self._dispose(mlist, msg, msgdata)
  File "/usr/local/lib/python3.6/site-packages/mailman/runners/pipeline.py", line 37, in _dispose
    process(mlist, msg, msgdata, pipeline)
  File "/usr/local/lib/python3.6/site-packages/mailman/core/pipelines.py", line 50, in process
    handler.process(mlist, msg, msgdata)
  File "/usr/local/lib/python3.6/site-packages/mailman/handlers/subject_prefix.py", line 180, in process
    mlist, msgdata, subject, prefix, prefix_pattern, ws)
  File "/usr/local/lib/python3.6/site-packages/mailman/handlers/subject_prefix.py", line 75, in all_same_charset
    chunks.append(chunk.decode(charset))
LookupError: unknown encoding: unknown-8bit

Please note that for test reasons I am currently BCC'ing mails to a single Mailman 3 list with only one menber. The list is set to Accept immediately (bypass other rules), to avoid complains about "Message has implicit destination" as a consequence.