electrical circuit conditions

Hi,

i'm tring to studing the current distribution and juole losses in an indutor. I work with ac voltage sourge of 1000V with a frequency of 20kHz. i use an impedence boundary condition because my skin dept is so small. In my model i work with a lumped port connected with an electrical circuit. In the electrical circuit i define an voltage sorce and then an external I vs U (linked with my fem model). The model doesn't work and i get the the following error:"System matrix is zero".

Can someone help me?

Thanks
Nicola