Fields from Zoho CRM not appearing in merged document

Fields from Zoho CRM not appearing in merged document

Hi,

I have a custom CRM module where certain fields are looked up from "Contacts". 

When I then try to run a mail merge from my custom module, the date from the fields which were fetched from contacts do not appear in the document. I just see the field ID. 

Does anyone have any idea how to fix this?

Thanks