message.as_bytes() maximum recursion depth exceeded

Traceback (most recent call last):
  File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.4/dist-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
    ....
  File "./utils/mailer.py", line 98, in send_multiple_tickets
    send_async_email(msg)
  File "./utils/mailer.py", line 25, in send_async_email
    mail.send(msg)
  File "/usr/local/lib/python3.4/dist-packages/flask_mail.py", line 492, in send
    message.send(connection)
  File "/usr/local/lib/python3.4/dist-packages/flask_mail.py", line 427, in send
    connection.send(self)
  File "/usr/local/lib/python3.4/dist-packages/flask_mail.py", line 190, in send
    message.as_bytes() if PY3 else message.as_string(),
  File "/usr/local/lib/python3.4/dist-packages/flask_mail.py", line 385, in as_bytes
    return self._message().as_bytes()
  File "/usr/lib/python3.4/email/message.py", line 178, in as_bytes
    g.flatten(self, unixfrom=unixfrom)
  File "/usr/lib/python3.4/email/generator.py", line 109, in flatten
    self._write(msg)
  File "/usr/lib/python3.4/email/generator.py", line 175, in _write
    self._dispatch(msg)
  File "/usr/lib/python3.4/email/generator.py", line 208, in _dispatch
    meth(msg)
  File "/usr/lib/python3.4/email/generator.py", line 266, in _handle_multipart
    g.flatten(part, unixfrom=False, linesep=self._NL)
  File "/usr/lib/python3.4/email/generator.py", line 109, in flatten
    self._write(msg)
  File "/usr/lib/python3.4/email/generator.py", line 189, in _write
    self._write_headers(msg)
  File "/usr/lib/python3.4/email/generator.py", line 416, in _write_headers
    self._fp.write(self.policy.fold_binary(h, v))
  File "/usr/lib/python3.4/email/policy.py", line 179, in fold_binary
    folded = self._fold(name, value, refold_binary=self.cte_type=='7bit')
  File "/usr/lib/python3.4/email/policy.py", line 192, in _fold
    return self.header_factory(name, ''.join(lines)).fold(policy=self)
  File "/usr/lib/python3.4/email/headerregistry.py", line 252, in fold
    return header.fold(policy=policy)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 299, in fold
    self._fold(folded)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 1205, in _fold
    rest._fold(folded)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 345, in _fold
    if folded.append_if_fits(part):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
    ... many fucking lines removed ...
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 336, in _fold
    if folded.append_if_fits(part, tstr):
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 148, in append_if_fits
    token._fold(self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 323, in _fold
    tstr = str(part)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 195, in __str__
    return ''.join(str(x) for x in self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 195, in <genexpr>
    return ''.join(str(x) for x in self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 195, in __str__
    return ''.join(str(x) for x in self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 195, in <genexpr>
    return ''.join(str(x) for x in self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 195, in __str__
    return ''.join(str(x) for x in self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 195, in <genexpr>
    return ''.join(str(x) for x in self)
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 629, in __str__
    return quote_string(''.join(str(x) for x in self))
  File "/usr/lib/python3.4/email/_header_value_parser.py", line 629, in <genexpr>
    return quote_string(''.join(str(x) for x in self))
RuntimeError: maximum recursion depth exceeded while calling a Python object