Set salesRep to NULL

I'm adding leads to Netsuite via my own web form. The trouble is, Netsuite is automatically populating the "salesRep" field.

I've tried putting this in the nullFieldList, but no joy.

How do I add a lead without assigning a salesRep? Here's where I'm at:

// Add the customer
$customerFields = array (
[INDENT]'isPerson'     => true,
'firstName'     => $FORM_FIRST,
'lastName'     => $FORM_LAST,
'companyName'    => $FORM_COMPANY,
'phone'      => $FORM_PHONE,
'email'      => $FORM_EMAIL,
'globalSubscriptionStatus' => $FORM_OPTIN,
'entityStatus'    => array('internalId' => '6'),
[/INDENT]);

$nullFieldArray = array('salesRep');
$customerFields['nullFieldList'] = $nullFieldArray;

$customer = new Customer();
setFields($customer, $customerFields);

$request = new AddRequest();