Optional JOI object if certain value is selected

How would I make an optional/required object based on another value?

I would like extra to be required if dessert is equal to Cheese. If it's set to cake extra should not be present.

{
  dessert: Joi.string().valid('Cake', 'Cheese').required(),
  extra: Joi.object({
    grapes: Joi.string(),
    wine: Joi.string(),
  })
}