How to solve first-order PDE?

Hello,

I try to solve 2D first order PDE in COMSOL, but unsuccessful yet. The equation is f_t + x*f_x + y*f_y = 0 with initial condition f(x,y,t=0) = y/x - log(x) on square area S = (1..9, 0..1). The analytic solution looks like f(x,y,t) = y/x + log(x) + t, but COMSOL gives wrong solution. I use Convection-Diffusion module.

Any thoughts,
Thanks.