Singular matrix error

Hi,

I am designing a simple 2D ridge waveguide structure on comsol but i am getting an error '' There are 1 void equations (empty rows in matrix) for the variable comp1.emw.S1x. at coordinates: (0,0), ... There are 1 void equations (empty rows in matrix) for the variable comp1.emw.S2x. at coordinates: (0,0), ... and similarly for the degrees of freedom (empty columns in matrix).''
Can anyone please help me to know the reason ?

Thanks