当我添加.when()时，Node Express joi验证失败

const Joi = require('joi');
const express = require('express');

const app = express();

app.post('/api/user', (req, res) => {
  const schema = Joi.object({
    username: Joi.string().required(),
    password: Joi.string().required(),
    confirmPassword: Joi.string().required().when('password', {
      is: Joi.exist(),
      then: Joi.valid(Joi.ref('password')).required(),
      otherwise: Joi.optional()
    })
  });

  const { error, value } = schema.validate(req.body);
  if (error) {
    // 处理验证失败的情况
    res.status(400).send(error.details[0].message);
    return;
  }

  // 处理验证成功的情况
  // ...
});

app.listen(3000, () => {
  console.log('Server started on port 3000');
});