Undefined value found

I had great trouble meshing a geometry. After I refined the mesh and was finally able to mesh it, I clicked the compute button. 78 seconds later, I got this message:

Undefined value found.
- Detail: Undefined value found in the equation residual vector.
.There are 1 degrees of freedom giving NaN/Inf in the vector for the variable mod1.emw.S1x
. at coordinates: (0,0,0), ...
There are 1 degrees of freedom giving NaN/Inf in the vector for the variable mod1.emw.S2x
. at coordinates: (0,0,0), ...

There is also a warning:

Inverted mesh element near coordinates (1.08012e-007, 3.25e-008, 6.4316e-007).

Can anyone help me with this? What's the problem? What should I do?