NJsonSchema reports validation error on a missing constrained but not required array

Hi,

I am using NJosnSchema to validate json events for an API. One of schemas provided by API is this. Note it has an array field 'prohibited' which is NOT marked as required, but has a constraint imposed on it - if it's present, it must have at least one item.

Now, I try to validate this json using that schema - and it presents me with a validation error: ArrayExpected: #/message.prohibited

I'd expect that should not be the case?