If I want to calculate the potential energy at the earth's center.. what's the rule?

Potential energy in a the earth's gravitational field is PE= -GMm/r, where M is the earth's mass. For a point that is inside the earth the value for M is the mass of the sphere below that point, so for a point at the center of the earth M=0. But r = 0 as well, so this looks like an eqaution that has 0 divided by 0, so undefined. However, we can substitute , so potential energy becomes:



which is 0 at r=0.