First name: [FirstName,fallback=] ...not working

I have the latest version of Sendy (1.1.6.3). I'm trying to create the [FirstName,fallback=] Personalization tag and have not succeeded in getting it to work.

I've created the custom field [FirstName,fallback=] and then put that same tag into a compose mode of a real (not test) mail blast. It came up empty with the first name. I tested the [Name,fallback=] and it works fine.

Where is there documentation on creating the FirstName Personalization tag? I see no example on how to do this.