HFSS port 1 does not have a solved inside material on either side

In the attached model I'm getting the error message "port 1 does not have a solved inside material on either side". The port is a lumped port, which connects two pec conductors. To the

best of my knowledge, the model has vacuum outside that. Can anyone suggest what is wrong with the model.

Note, the model uses FR4, but I only chose this as FR4 printed circuit board is a fibreglass material, and the antenna in the model will be surrounded by a fibreglass radome. Perhaps it might have been less confusing if I had added a material "fibreglass", which has the same properties as FR4.