externalDocs without url property

I have a use case where I'd like to use the externalDocs object without specifying a url (basically I want to indicate that some attachment content exists, but don't want to provide any additional details on the attachment iself for security/privacy reasons). Right now OpenAPI 2.0 throws an error saying the url property is required. Does anyone have a workaround or have a suggestion for another way to represent my attachment?