Object with 'or', keys contains an array with a 'null' element

[gagle]

joi.object().keys({
  a: joi.string(),
  b: joi.string()
}).or('a', 'b')

This is what I get if 'a' and 'b' doen't exist:

"validation": {
  "source":"payload",
  "keys": ["null"]
}

Should keys be an empty array? I'm using the master branch.

[Marsup]

Thanks for the report, now it's more aligned with other validations.