Help with simple popup script on field change

Hi,

I need a simple script to pop a screen message if a custom entity (date) field called custentitypaidlicense, is changed from blank to a date. Here's what I had:

function licenseFieldChanged()
{
if ( nlapiGetFieldValue('custentitypaidlicense') == '' )
{
alert('Once the license fee has been paid, please email Operations');
return false;
}
return true;
}

This seems to partially work, but something must be missing as it does not work consistently. Any help would be appreciated.