What's the meaning of this error?

Undefined value found.
- Detail: Undefined value found in the stiffness matrix.
There are 50197 equations giving NaN/Inf in the matrix rows for the variable mod1.theta
at coordinates: (0,0), (0,1e-007), (0,2e-007), (6.51016e-008,1.67803e-007), (1.30203e-007,1.35606e-007), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns

ps: theta is a dependent variable

Thank you !