Validate key without making it required?

[marcysutton]

I want to validate a schema to catch incorrect keys while keeping them optional. I've looked through the options, but I'm not sure if this is possible.

Here's a small portion of my schema for accessibility checks, I want to validate the key incomplete while keeping it optional:

messages: {
	required: true,
	type: 'object',
	properties: {
		fail: {
			required: true,
			type: 'string'
		},
		pass: {
			required: true,
			type: 'string'
		},
		incomplete: {
			type: ['string', 'object']
		}
	}
}

The only thing I've gotten revalidator to complain about is if I make incomplete required. The motivation for this change is I want to prevent check authors from using undefined: {} instead of incomplete: {}, where only some checks will have incomplete messages specified (hence the optional bit).