I am creating a package on the Account object and need to pull information from the Account Assignment related list and am running into a problem. I've searched and read several articles & how-tos without finding exactly what I'm looking for.

I created a relationship on the package.  However, when I attempt to add the Default Value on the form by search button and selecting Account Assignment from the picklist I receive the following error:  

The value 'null' is not valid for operator '>='