Why did I get error with NaN/Inf ?

Hello, I am running a simulation for a solid oxide fuel cell, with three physics interfaces, accounting for fluid flow, mass transport and current distribution. But I can not get it converged, so I started decoupling the system, and start with only solving for fluid flow and current distribution. My model can iterate for several iterations, but will give some error msg like :

Undefined value found.
- Detail: Undefined value found in the equation residual vector.
.There are 52449 degrees of freedom giving NaN/Inf in the vector for the
variable mod1.XX

I wonder if anyone can help explain the reason so that I can correct my model? thanks!