Error Field Not Found

Hi,

I have a custom field that creates a default salutation for our customers based off of several criteria. This field needs to work for customers (individuals and companies) as well as contacts. The formula works great for customers, but I get a field not found error on contact records due to the fact that {altname} doesn't exist. I essentially just want to skip this statement on contact records. How do I check if the field exists in a way that doesn't produce an error?

CASE WHEN {custentity_salutation_formal} is NULL
THEN CASE WHEN {lastname} is NULL
             THEN {altname}