I'm trying to implement the following validation rule using Joi.

Basically I have 2 properties a and b, I'd like to check that a is less than or equal to b only if b is not undefined.

Here is what I've tried so far:

const mySchema = Joi.object().keys({
    a: Joi.number().concat(Joi.number().when('b', { is: Joi.not(undefined), then: Joi.number().max(Joi.ref('b')).message('a should be less than or equal to b') })
    b: Joi.number() })

I get the following error message when testing the schema:

Cannot call allow/valid/invalid with undefined