In a computed field, how do I count the number of items where a pick list is unspecified?

I have a picklist that is not required on an item and I want to create a computed field to count the number of items where the picklist has no value yet.  I tried to use the "Unspecified" value, but the following error is returned: 

MKS124543: The field Result is a picklist field. It may only be compared against a picklist value from that field. Unspecified is not a known picklist value.

Thank you,

Paul Hartwig