Comsol 4.3b_ Heat Transfer Module

I am doing simulation with Heat transfer module/Joule heating and I get the convergence error of " Failed to find a solution. Divergence of the linear iterations. There was an error message from the linear solver. Very ill-conditioned preconditioner. The relative residual is more than 1000 times larger than the relative tolerance.
Returned solution is not converged."
It is a simple model; I applied a potential to a silver surface connected to one side of a carbon film, while the other side of the film connected to the ground by another silver surface. I thought maybe some boundary conditions must be defined that I am not aware of them. Does any body have any idea?

Regards,
Elham