Nulls getting passed to the database on insert?

I've got an entity that only includes three fields from a database table.  When I create a new entity, populate my three fields, and then save, I get an error: "can't save null value in field X".  This field 1) isn't included in my business object, 2) isn't nullable,  and 3) has a default value in the database.

Is DevForce trying to set it to null anyway?  What is the best solution for this?

Thanks!