I'm creating a part with lots of configurations to account for different sizes and materials.  I've created six different configurations, each of which has a different material.  When I create a design table, it puts in a field for material and everything is happy.  Any time I try to edit the design table, however, I get a bubble that says, "The value "materials:Buna-N" in column "\$Library:Material@o-ring" is invalid."  It then undoes all of the edits that I've put in the table.  It seems to be unhappy with the materials column that IT created.

Any ideas what the problem is and how to correct it?

Thanks in advance for your suggestions.