hfss+material assignment

I have a 2d simulation that i am trying to troubleshoot, I keep getting this error: at least one material assignment should have solved inside set

What does this mean? I'm hoping someone can point me in the right direction..

-js

May be you are need set some material to your inside object. if you define boundary condition (vacuum,air) will have to set another material(not air or vacuum) to inside object