On a registration form I have a single input called Full Name. The strings are parsed with http://code.google.com/p/php-name-parser/ so if a person enters their full name middle or any quantity of strings it's handled just fine and the app creates the user in a billing system with it's API.

The form validates and checks for two strings in the field otherwise it won't post. I'd like to remove this validation but a last name is required by the API. You cannot post an empty last name to the API.

Users are signing up for a trial so I don't want them having to deal with many form fields. The only place the last name shows up visible to the user is in their account settings page. If they end their trial and start a paid plan they'd have to enter their billing details which asks with two fields for their First, Last, and other billing information.

What is an alternative to submitting "Doe", "Default", "Empty" in place of them not filling in their last name?