Cost Centers and the Expense Account

Hi,

I created a simple client code which is checking if the account on the Journal Entry is Expense Account and if the Departments is a Cost Center and matches them.
If they do not match, alert is returned.
Script is doing it's job, but alsways when i press a Tab after selecting the Account, an error message is displayed but the code still works:
SSS_MISSING_REQD_ARGUMENT
nlapiLookupField: Missing a required argument: id

Here is the code:

function checkCostCenter(type,name,linenum)  {  var i = nlapiGetCurrentLineItemIndex('line'); //get linenum   nlapiSelectLineItem('line',i); //select the line   {  var thisAccount = nlapiGetCurrentLineItemValue('line','account'); // specifiy the internalId of the Account field value  var isExpense = nlapiLookupField('account', thisAccount, 'custrecord10'); // find the custom field value    var thisDept = nlapiGetCurrentLineItemValue('line','department');// specifiy the internalId of the Department field value  var isCostCenter=nlapiLookupField('department', thisDept, 'custrecord7'); // find the custom field value    if((isExpense=='F'&& isCostCenter != '')|| (isExpense=='T'&& isCostCenter == '')) {    alert('You cannot connect Non-Expense Account with Cost Center! Please change an Account or Department value!');  return false;  }  return true;  }  }