I was doing a problem where we have a circular disk of radius $b$ and a grounded spherical conducting shell of radius $a < b$. The task is to find the electric potential everywhere. I was going to solve it by finding the Green function everywhere and then integrating, but when I was going to find the Green function inside the shell my professor said the potential in there was zero. Why? I thought that you could only discard the electric potential like that when the potential is zero at the boundaries and when there are no charges in the region. In this case there is a test charge so one wouldnt be able to make that argument. Where am I wrong?