Step index fiber

Hi,
In the simulation of step-index fiber if i place a particle beside the core and want to evaluate a line integration of the
following equation ----------------
((-conj(emw.Dz)*emw.Ex-conj(emw.Bz)*emw.Hx)*nx+(-conj(emw.Dz)*emw.Ey-conj(emw.Bz)*emw.Hy)*ny+(.5*(conj(emw.Dx)*emw.Ex+conj(emw.Dy)*emw.Ey+conj(emw.Dz)*emw.Ez+conj(emw.Bx)*emw.Hx+conj(emw.By)*emw.Hy+conj(emw.Bz)*emw.Hz)-conj(emw.Dz)*emw.Ez-conj(emw.Bz)*emw.Hz)*nz)

there arises an error saying that .......
"Failed to evaluate variable nz".

Is it because, i am not taking 3D model ???
Is there any way to incorporate this equation into 2D model?