SuiteScript Error On Drop Ship Items

I am using a scheduled script to write values to a couple of custom fields on inventory item records.  It works great for most items.  However, when writing to items which are marked as special order items - I get the error message "Drop ship/Special order items must have a preferred vendor and purchase price."

I've triple checked these item records and there is already a preferred vendor & vendor price on the vendor sublist.

Following is a code snippet.  I've tried the do Sourcing parameter both ways and neither worked.  Any thoughts?

            var fields = ['custitem_last_sales_order', 'custitem_last_so_date'];