Strange Error On WorkflowAction Script

Hi,

I have a script which loads at a variable with a value from a List/Record Transaction field, I have got this value coming into my script, I then do a IF (variable !== null) {}

The problem I have is if I run this through the debugger it works perfectly when there is a null value in the field this part of the script doesn't exceute, if I run it from a button as a workflowaction I get a record does not exist error at that point.

Can't understand why?  Code snippet below

 var newRec = nlapiGetNewRecord();    nlapiLogExecution('DEBUG', 'New Record Internal ID', newRec.getId());      //var newRec = nlapiLoadRecord('customrecord_wpa_20',24);   //For Testing         var POid = newRec.getFieldValue('custrecord_wpa_contractor_po');            //Capture the subcontractor PO Number    nlapiLogExecution('DEBUG', 'PO ID', POid);      var Soid = newRec.getFieldValue('custrecord_wpa_so');               //Get SO Number for Transform     nlapiLogExecution('DEBUG', 'SO ID', Soid);      //var subCheck = newRec.getFieldValue('custrecord_wpa_data_con_checkbox');            if (Soid !== null)     {    // Close the attached Sales Order