通过Python电子邮件库发送电子邮件会抛出错误"expected or bytes-like object“

import smtplib
from email.mime.text import MIMEText

def send_email(sender, receiver, subject, message):
    # 创建邮件内容
    msg = MIMEText(message, 'plain', 'utf-8')
    msg['From'] = sender
    msg['To'] = receiver
    msg['Subject'] = subject

    try:
        # 连接SMTP服务器
        smtp_server = 'smtp.example.com'  # 替换为实际的SMTP服务器地址
        smtp_port = 587  # 替换为实际的SMTP服务器端口
        smtp_username = 'your_username'  # 替换为实际的SMTP用户名
        smtp_password = 'your_password'  # 替换为实际的SMTP密码
        smtp_connection = smtplib.SMTP(smtp_server, smtp_port)
        smtp_connection.starttls()
        smtp_connection.login(smtp_username, smtp_password)

        # 发送邮件
        smtp_connection.sendmail(sender, receiver, msg.as_string())
        smtp_connection.quit()
        print("邮件发送成功！")
    except Exception as e:
        print("邮件发送失败:", str(e))

# 调用发送邮件函数
send_email('sender@example.com', 'receiver@example.com', '测试邮件', '这是一封测试邮件。')