FD solver in CST Mws

Hi

I designed a nanoantenna structure in CST and defined a plane wave for excitation and boundary condition is set to open add space. when I try to run FD solver for it, program gives an error about fd solver not working with open boundaries . is it common or I missed some settings somewhere ?
cause I've read that fd solver is the suitable solver for working with high frequencies in cst.

thanks in advance.