BOM Description column driven by equation

I have a solidworks fixture drawing acquired from one of our vendors.  I'm trying to use toolbox screws/dowels.  The part models have a "Description" property that I want to use in a column, but the screws/dowels don't have that property, or its blank.  For screws/dowels, I want to use "PartNumber".  Here's my if statement:

IF(`Description`="";`PartNumber`;`Description`)

Except the solidworks table only returns "Description"s, and the hardware rows are blank in this column.  If I change "Part Number" to "swFileName", then all rows populate as expected, with part model rows reporting their "Description" and toolbox model rows reporting their filename.  So my conditional is correct.  It seems that for some reason "PartNumber" cannot be used in an "if" equation.  Can anyone confirm this?  Am I missing something obvious?

SolidworksBill Of Materials