Problems with nlapiSearchDuplicate on customer form

Hi,
I have a problem: In my custom customer form, i've custom field. This field is a mandatory field: custentity4.
I want to validate this field from suitescript, but when i execute this script on my custom customer form, i've this error:

"An unexpected error occurred in a script running on this page.
validaCamposBody (validateField)
customform
SSS_INVALID_SRCH_FILTER
An nlobjSearchFilter contains invalid search criteria: 3."

My SuiteScript is:

function validaCamposBody(type, name)
{
//Valida duplicidad de clientes por campo RUC
if (name == 'custentity4')
{
  var arraySearch = new Array();
  arraySearch[0] = nlapiGetFieldValue('custentity4');
  var duplicateRUC = nlapiSearchDuplicate('customer',arraySearch);
 
 
  if (duplicateRUC.lenght >= 1)