How to do a PDM Search for a NULL variable value

I am needing to find all drawings that contain a NULL value for the property Part Number.

This does not seem to work:

And

It seems to ignore the variable criteria.  BTW, Part Number is a text field.  I could also use the Revision property, which is an integer, but there is still not a way to find just the BLANK values.  

Thank you.

SolidworksSolidworks Pdm/enterprise Pdm