COMSOL error

Hi,

When I do the coupling calculation for laminar flow module and transport and diluted species module (concentration), it always shows an error:

Undefined value found.
- Detail: Undefined value found in the stiffness matrix.
There are 11 equations giving NaN/Inf in the matrix rows for the variable comp1.c.
at coordinates: (-0.00124375,0.00168601), (-0.00124375,0.00168501), (-0.00124375,0.00168401), (-0.00124375,0.00168301), (-0.00124375,0.00168201), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns.

Do you know what it is? How to avoid this happen?

Thanks,