Creating Task - Script

Hi,

I am trying to run a script after an invoice is created that, based on a field be ing checked or not, creates a task due in 30 days..... However I keep getting this error:

Error: INVALID_REF_KEY

Invalid reference key [null].

Stack Trace....

Any help would be greatly appreciated... I'm not sure what's going on...

function FollowUp()
{
   
    if (nlapiGetFieldValue('custbody_check_box') == 'F')
    {

var dDate = nlapiGetFieldValue('startdate') + 30;
var sRep = nlapiGetFieldValue('salesrep');
var cID = nlapiGetFieldValue('entity');
var tID = nlapiGetFieldValue('tranid');

var recordCreated = nlapiCreateRecord('task');
recordCreated.setFieldValue('title', 'Follow-up Required' );
recordCreated.setFieldValue('assigned', sRep);
recordCreated.setFieldValue('company', cID);
recordCreated.setFieldValue('duedate', dDate);