Uncaught runner exception: 'ascii' codec can't encode character '\ufffd' in position 2013: ordinal not in range(128)

GNU Mailman 3.2.0a1.

Jan 16 10:13:39 2018 (36) Uncaught runner exception: 'ascii' codec can't encode character '\ufffd' in position 2016: ordinal not in range(128)
Jan 16 10:13:39 2018 (36) 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/outgoing.py", line 93, in _dispose
    self._func(mlist, msg, msgdata)
  File "/usr/local/lib/python3.6/site-packages/mailman/mta/deliver.py", line 86, in deliver
    refused = agent.deliver(mlist, msg, msgdata)
  File "/usr/local/lib/python3.6/site-packages/mailman/mta/bulk.py", line 101, in deliver
    mlist, msg, msgdata, recipients)
  File "/usr/local/lib/python3.6/site-packages/mailman/mta/base.py", line 70, in _deliver_to_recipients
    sender, sorted(recipients), msg.as_string())
  File "/usr/local/lib/python3.6/site-packages/mailman/mta/connection.py", line 84, in sendmail
    results = self._connection.sendmail(envsender, recipients, msgtext)
  File "/usr/local/lib/python3.6/smtplib.py", line 855, in sendmail
    msg = _fix_eols(msg).encode('ascii')
UnicodeEncodeError: 'ascii' codec can't encode character '\ufffd' in position 2016: ordinal not in range(128)

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.