context.oldRecord is getting null value

In a workflow action script, I used context.oldRecord and it's getting a null value and throwing an error.

I deployed the workflow action script on the vendor prepayment and used it in a workflow.

/**

 * @NApiVersion 2.x

 * @NScriptType WorkflowActionScript

 */

 

define(['N/search','N/record'], function (search,record) {

 function onAction(context) 

 {

var oldRec = context.oldRecord;

var newRec = context.newRecord;

log.debug({title: 'Record created',details: 'oldRec==: '+oldRec });

log.debug({title: 'Record created',details: 'newRec==: '+newRec });

// Check Approval Just Changed

var oldStatus = oldRec.getValue({ fieldId: 'approvalstatus' });

var newStatus = newRec.getValue({fieldId: 'approvalstatus' });

log.debug({title: 'Record created',details: 'oldStatus==: '+oldStatus });