NaN or Inf found when solving linear system using SOR in Double nano-Granting

Hi everyone,

I am trying to simulate the intensity that a light beam through a double diffraction grating.However, when I run the computation, an error of "UNDEFINED VALUE FOUND" occurs, with detail "NAN OR INF FOUND WHEN SOLVING LINEAR SYSTEM USING SOR". - Feature: Stationary Solver 1 (sol3/s1)". I don't know where exactly the undefined value is coming from. How can I avoid/fix this error?

Undefined value found.
- Detail: NaN or Inf found when solving linear system using SOR.
Could anyone help me figure out what may cause the error? Due to extension error, I am not able to attach my model to this message.

Thanks in advance.
宾 王