Invalid entitystatus reference key 6.

I am trying to create a new Customer with an entity status (6, Lead-Unqualified) with a Suitelet. We have a customform called "Sales Lead Form" which allows us to assign the Status of a customer to Lead-UNQUALIFIED.

However, when I execute a suitelet I'm getting the following error message.

Invalid entitystatus reference key 6.

Can anyone shed light on this? I'm thinking it's the custom form that's causing this error, but not sure how to go about debugging the problem. Here is the sample code:

function CreateCustomer(request, response)   {

    if (request.getMethod() == 'POST') {
    
     var customer = nlapiCreateRecord('customer');
     customer.setFieldValue('isperson', 'T');
     customer.setFieldValue('customform', '3'); // Standard Lead Form