emailMerger.setEntity() Seems to be not merging the used template with the entity

Hi,
I'm trying to send a template mail on user event to a customer in my sandbox and below is my code :

            var emailMerger = nlapiCreateEmailMerger('22');
            emailMerger.setEntity('customer', customerId);
//            emailMerger.setRecipient('customer', customerId);
            var mergeResult = emailMerger.merge();
            var emailSubject = mergeResult.getSubject();
            var emailBody = mergeResult.getBody();


            emailBody = emailBody.replace('{1}', name);
            emailBody = emailBody.replace('{2}', date);
           
            nlapiSendEmail('20', 'user1@mytestserver.com', 'Test Mail',emailBody, null, null, null, null);

But unfortunately, I'm unable to see any mail merged under the communication tab in that customer. Is anything I'm missing here ?