the problem about HFSS 9.2

When using HFSS to calculate the characteristic impedance of CPW,
at low frequency, such as 1MHz, it will get give some ridiculous results,
anyone has such experience? but at higher frequency, 100Mhz, everything is fine, I cant understand that is why.

Answer lies in the name HFSS High Frequency Structure Simulation dont use at a lower frequency