Vs2010 Data Generation Plan fails with “Data generation failed because of the following exception: Column ”xyz“ does not allow DBNull.Value”

I'm fairly new to Vs Data capabilities, and this is my first data generation plan. I have implemented a database using a Vs2010 database project, and used it to deploy to a sql server express 2008 database. All the tables use identity columns as their primary keys, and they're related to one another with foreign keys.

I set up a data generation plan, but when I try to generate data with it, the tables are simply populated in alphabetical order, which is of course going to fail. The only tables that populate correctly are the lookup tables and other sorts of independent entities with no FK constraints. The rest are skipped after the first table fails.

Supposedly the generation plan determines the population order based on FK dependencies. What happened?

edit: someone with the rep for it should make a visual-studio-data-tools tag, since DBPro is no longer (nor really ever was) a product name.