HFSS 18.0 - Port refinement, process hf3d error: Too few conductors were found on por

I am having a problem with my simulation: "Project1"
Project1_Simulation (Transient Network)
[error] Port refinement, process hf3d error: Too few conductors were found on port 1. There should be one conductor for each terminal, with one additional reference conductor. Unintentional contact between conductors may cause this error.. (9:38:07 PM Apr 17, 2017)"

I have confirmed that the two conductors defined on the port (ground plane and signal line) are not in contact.

I am an having a problem with an HFSS microstrip simulation: "Transient (Transient Network)
[error] Port refinement, process hf3d error: Port '1' contacts more than one non-conducting material which violates the TEM terminal modes restriction.."

My port, indeed, contacts both FR-4 and vacuum, but this doesn't seem to be an issue according to the documentation. Can anyone tell me how to avoid this?