Problem trying to access nlapiGetOldRecord within a script

I'm trying to write a script which will let me perform an action if the value of a field in a contact record is changed. I'm trying to use nlapiGetOldRecord().getFieldValue() to access the field value prior to a write operation and compare it to the field value returned by nlapiGetNewRecord().getFieldValue().

nlapiGetNewRecord().getFieldValue() works fine, the nlapiGetOldRecord().getFieldValue() method however returns the following error:

TypeError: Cannot call method "getFieldValue" of null (casNotesCreation.js#8)

Am I misinterpreting how the nlapiGetOldRecord API should work? Here is my whole script, which is called as an After Submit function.

function casCallActivity_ContactAfterSubmit(type)  {    //Only execute the logic if a new contact is created or if an existing contact is updated    if((type == 'create') || (type == 'edit') )    {     // Obtain a handle to the old contact record     var oldContRec = nlapiGetOldRecord();     var oldValue = oldContRec.getFieldValue('custentitymarketocasnotes');           //Obtain a handle to the new contact record      var newContRec = nlapiGetNewRecord();    var newValue = newContRec.getFieldValue('custentitymarketocasnotes');       nlapiLogExecution('DEBUG','<Before Submit Script> CASNotes New:'+ newValue + ' CASNotes Old:' + oldValue);        }  }