I am struggling with the syntax for providing a default value to an empty and/or missing array key.

Joi.object({
    items: Joi.array().empty([]).items(Joi.number()).default([42]),
})

{ 
    items: []
}
or
{
}

In both cases I would want the default value of [42] to be substituted into the object. I'm assuming that I have to indicate that an empty array is invalid in some way, but I'm not sure how. I have also tried setting min(1) on items, but that doesn't work either (it flags the object as invalid, but does not substitute the default value).