come across such a problem

I use the PDEs module to make simulation of my model, but I just encountered with a few problems of which is

undifined value in the stiffness matrix. I make sure that I have already defined all the dependent variables, but the software

point out that

"Undefined value found in the stiffness matrix.

There are 2001 equations giving NaN/Inf in the matrix rowsfor the variable mod1.ne at coordinates : (0), (5e-7), (1e-

006), (1.5e-006), (2e-006), …

There are 2001 equations giving NaN/Inf in the matrix rowsfor the variable mod1.ni at coordinates : (0), (5e-7), (1e-

006), (1.5e-006), (2e-006), …

and similarly for the degrees of freedom, NaN/Inf in the matrix columns. "

The mod1.ne and mod1.ni are the dependent variables. I don't understand the NaN/Inf and don't have any thought of

the problem I came across. Much thanks for your help!