Low Frequency Issues with CST MWS

All,
I am simulating a simple microstrip trace in CST. I am using discrete ports and PML (open + add space) boundary conditions. I notice that S12 does not quite reach 0dB at DC, even with PEC models for the conductors and no dielectric loss.
Is there something I should be doing to get S21 -> 0dB?
Thanks
Ed

Do you have Line Impedance Adaptation turned on? Do you have Inhomogenous Port Correction turned on? Both are in the solver settings... forgot the exact names...