Adding finite ground plane to quarter-wavelength monopole antenna

I have designed a monopole antenna for 30 MHz with length 3m and radius 0.01m and waveguide port in CST MWS. For adding the port, I used teflon around monopole antenna upto the height 0.05m. When I add a ground plane (PEC) of radius 2m and thickness 0.002m below the monopole antenna it gives the following error:-

"Port 1 is inhomogeneously filled within the first 3 mesh cells. Cannot start solver."

I have tried the following combinations:-
1. moving ground above port plane.
2. making ground plane with inner radius 0.05m.
3. moving ground plane further below the port plane.
4. changing mesh settings.

The error goes away only when I remove the ground plane. Please tell me how to place the ground plane correctly.