CST： Eigenmode solver error

Hello, all,
I'm trying to plot dispersion diagram with CST. I need to use eigenmode solver, but there is always on error: Eigenmode solver does not support lossy metal materials. The lossy metal materials will be treated as PEC.
Here the lossy material is graphene. Trying to fix this, I changed the imaginary part of permittivity of my material to 0. I think now it should be lossless, but the erro still occurs. Anyone could help? Thanks in advance.

for substrate material choose (LOSS FREE)
<iframe class="restrain" title="YouTube video player" width="640" height="390" src="http://www.youtube.com/embed/1OrBgJ2QsDM?wmode=opaque" allowfullscreen="true" frameborder="0"></iframe>