How to query workitems having an "unassigned" value of a custom attribute

We have a custom attribute (Eg: Found On) is created using an enumeration of values including the "Unassigned", the Default and Unassigned literal is set to "Unassigned" in the enumeration. Now we are writing a query to filter the workitems that are having value "Unassigned" for this attribute. If we add other conditions such as Type and status, we are able to view multiple workitems having this attribute value "Unassigned" but when we add the condition for the attribute and choose "Unassigned" in the condition no workitems are displayed. I have tried removing the Unassigned Literal to None in Enumeration, but still no success. Please let us know how to overcome this issue and able to filter the workitems based on the attribute condition value is Unassigned. Let me know in case of any additional information.

Thanks,

Vinay