How do i return records that have no values in a field?

How do i return records that have no values in a field?

Hi - i'm trying to return all records from a custom module that do not have a sales order number.

This is my code (which returns all records, including those with sales order numbers):


searchParam = {"cf_salesordernumber":""};
getNewSO = zoho.books.getRecords("cm_sor",organizationID,searchParam,"zohobooks");

What am I doing wrong?  Thanks!