Joi nested when validation

Question

I am trying to validate a nested object conditionally based upon a value in the parent.

const schema = Joi.object({
    a: Joi.string(),
    b: Joi.object({
        c: Joi.when(Joi.ref('..a'), { is: 'foo', then: Joi.number().valid(1), otherwise: Joi.number().valid(2) }),
    }),
});

const obj = {
    a: 'foo',
    b: {
        c: 2,
    },
};

In this example, I want to get an error that c must be 1, but the validation passes. I've tried with and without references, but I clearly must be misunderstanding something fundamental about how Joi works. Any help?