Error Jacobi with user-defined function

I defined a function in Global Definition to describe the graded-index. However, when I used the Solve-Modal Analysis, the COMSOL popped a window to warn error in Jacobi matrix.

Plus, I suppose my function was working well, I am not sure whether it can be a assignment in Material dialog.

If I replace my function with a constant in index value setting, the model works well.

Thank you.