Python EmailMessage创建邮件草稿时触发AttributeError: 'list' object has no attribute 'encode'错误求助

我最近在实现邮件草稿创建功能时碰到了一个诡异的问题，明明传入的文本都是字符串，但调用msg.as_bytes()的时候却触发了列表没有encode方法的错误，实在搞不懂哪里出问题了，麻烦大家帮忙看看！

我的核心代码是这样的：

def _create_draft(sender, to, subject, msg_html, msg_plain, attachments):
    msg = EmailMessage()
    msg['Subject'] = subject
    msg['From'] = sender
    msg['To'] = to
    msg.attach(MIMEText(msg_plain, 'plain'))
    msg.attach(MIMEText(msg_html, 'html'))
    if attachments:
        msg = _add_attachments(msg, attachments)
    return {'raw': base64.urlsafe_b64encode(msg.as_bytes()).decode()}

触发的错误回溯信息如下：

File "/srv/www/code/project/email.py", line 56, in write_security_email
send_message(from_address, to_address, subject, html_body, txt_body)
File "/srv/www/code/project/gmail/gmail_api.py", line 52, in send_message
msg = _create_draft(sender, to, subject, msgHtml, msgPlain, attachments)
File "/srv/www/code/project/gmail/gmail_api.py", line 84, in _create_draft
return {'raw': base64.urlsafe_b64encode(msg.as_bytes()).decode()}
File "/usr/lib/python3.8/email/message.py", line 178, in as_bytes
g.flatten(self, unixfrom=unixfrom)
File "/usr/lib/python3.8/email/generator.py", line 116, in flatten
self._write(msg)
File "/usr/lib/python3.8/email/generator.py", line 181, in _write
self._dispatch(msg)
File "/usr/lib/python3.8/email/generator.py", line 218, in _dispatch
meth(msg)
File "/usr/lib/python3.8/email/generator.py", line 431, in _handle_text
if _has_surrogates(msg._payload) and not self.policy.cte_type=='7bit':
File "/usr/lib/python3.8/email/utils.py", line 57, in _has_surrogates
s.encode()
AttributeError: 'list' object has no attribute 'encode'

我观察到几个奇怪的现象：

• 当只发送纯文本邮件（不带附件）时，就会触发上面的错误。我确认msg_html和msg_plain都是字符串，不是列表，甚至把它们直接替换成固定字符串"my string"，还是会报错，而且attachments变量是None。
• 当发送带附件的邮件时，邮件能正常发送出去（但没有正文内容），不会触发错误。
• 如果把那两行msg.attach(MIMEText(...))注释掉，邮件能发送，但同样没有正文。
• 在我添加附件功能之前，这段代码是能正常发送带正文的邮件的，肯定是我加附件功能时改了什么地方但忘了，可就是找不到问题所在。

实在是百思不得其解，有没有大佬能帮我找找问题出在哪里呀？非常感谢！

备注：内容来源于stack exchange，提问作者David Murphy