I have a field in a JOI schema that I would like to be optional (i.e. undefined is accepted, and null is also accepted), however if a value for it is supplied, it must be a positive integer. How might I go about achieving this?

Here is what I have tried so far, with the field to validate being "capacity" however it does not seem to work, it appears the ".when" statement is just being ignored:

const divebarSchema = joi.object({
  divebar: joi
    .object({
      title: joi.string().required(),
      capacity: joi.optional().allow(null),
      description: joi.string().required(),
      location: joi.string().required(),
      image: joi.string().optional().allow(""),
      map: joi.string().optional().allow(""),
    })
    .required()
    .when(joi.object({ capacity: joi.exist() }), {
      then: joi.object({ capacity: joi.number().integer().min(0) }),
    }),
});

Before the above I originally had no .when and instead the capacity rule was:

  capacity: joi.number().optional().allow(null).integer().min(0),

However that also did not work, it kept throwing the error "must be a number" when submitting a null value.