First script ever, so apologies for being a rookie

I wanted to match a field on the customer record with one from a case. Both fields are drop downs and they use the same list.

I wrote this user event script that executes when the case is saved:

function leadBeforeSubmit(type){
nlapiLogExecution('DEBUG', 'Round 1', 'It started');
var rec = nlapiGetNewRecord();
var customerTag = rec.getFieldValue('customer.custentity34');
nlapiLogExecution('DEBUG', 'Round 2', 'Customer tag is '+customerTag);
if(customerTag == null){
  rec.setFieldValue('customer.custentity34' , 'custevent11');
  nlapiLogExecution('DEBUG', 'Round 3', 'done?');
}
else{
};
};

Everything works fine except:

  rec.setFieldValue('customer.custentity34' , 'custevent11');