Несвязанная ошибка наследования по умолчанию для переменных типа: почему?

Я делаю это так:

def send_mail
    msg['Subject'] = subject
    msg['From']    = sender
    msg['To']      = [email1@to.de,email2@to.de]
    msg['Cc']      = [email3@cc.de,email4@cc.de]
    msg['Bcc']     = [email5@bcc.de,email6@bcc.de]
    recips  = [email1@to.de,email2@to.de,email3@cc.de,email4@cc.de,email5@bcc.de,email6@bcc.de]
    sendmail(sender, recips, msg.as_string())


msg['From']    = sender
msg['To']      = [email1@to.de,email2@to.de]
msg['Cc']      = [email3@cc.de,email4@cc.de]
msg['Bcc']     = [email5@bcc.de,email6@bcc.de]
recips  = [email1@to.de,email2@to.de,email3@cc.de,email4@cc.de,email5@bcc.de,email6@bcc.de]
sendmail(sender, recips, msg.as_string())