Adding a property with the value in a "not set" state

[Sagara Paranagama]

Hi all,

How can I add a property with its value in a "not set" state?

My exact problem is that I have a base type, and it's parentId must be in a
"not set" state according to the CMIS specification. I tried not adding the
parentId property but the TCK reported "FAILIURE: Property cmis: parentId",
  "FAILIURE: Property not included in response".

How can I get around this?

Thanks.

Sagara