Test bypassing Joi validation (Nodejs course chapter 13)

I was following the course material and learning the integration test. In the 10th video “Testing Invalid Inputs”, my test keep failing and I can not find out why.
My genre code has validation function like the sample code:

function validateGenre(genre){
    const schema = {name: Joi.string().min(5).max(50).required()}
    return Joi.validate(genre, schema)

exports.validate = validateGenre

And the genres module has the post block like this:

router.post('/', auth, async (req, res) => {
    let {error} = validate(req.body)
    if (error) res.status(400).send(error.details[0].message)

    let genre = new Genre({name: req.body.name})
    await genre.save()

The test code:

describe('POST /', () => {
        it('should return 401 if client is not logged in', async () => {
            const res = await request(server).post('/api/genres')
                                             .send({name: 'genre1'})

        it('should return 400 if genre is less than 5 characters', async () => {
            const token = new User().generateAuthToken()
            const res = await request(server).post('/api/genres')
                                             .set('x-auth-token', token)
                                             .send({name: '1234'})

And the message of failed test:

  ● /api/genres › POST / › should return 400 if genre is less than 5 characters

    ValidationError: Genre validation failed: name: Path `name` (`1234`) is shorter than the minimum allowed length (5).

      at model.Object.<anonymous>.Document.invalidate (node_modules/mongoose/lib/document.js:3123:32)
      at node_modules/mongoose/lib/document.js:2911:17
      at node_modules/mongoose/lib/schematype.js:1349:9

Looks like the test bypassed Joi validation in the genre module and went into mongoose schema validation, which is not right. However, if I do it in Postman, it return error code 400 just fine.

Can anyone help me figure out why the test is failing? Thanks a lot!