Directional Coupler

I am trying to model a directional coupler in 2D. I get the following error
Undefined value found.
- Detail: Undefined value found in the equation residual vector.
There are 1 degrees of freedom giving NaN/Inf in the vector for the variable comp1.ewbe.S1x.
at coordinates: (0,0), ...
There are 1 degrees of freedom giving NaN/Inf in the vector for the variable comp1.ewbe.S2x.
at coordinates: (0,0), ...
There are 1 degrees of freedom giving NaN/Inf in the vector for the variable comp1.ewbe.S3x.
at coordinates: (0,0), ...

What does this error mean?