Characteristic is not available for filtering in BEx Query

Hi,

I have a navigational attribute in a query I want to put filter on. The value I am looking for is "Not assigned". When I do select filter value, I got all other values instead of "Not assigned" as if it does not exist. But, when I drill down on that navigational attribute I do see "Not assigned". The problem is that is just not available for filtering.

Can you please advice how to proceed further?