non-manifold edges hfss

Hi. I am new to HFSS and am encountering the following problem.

I am modeling two cascaded cavities and am getting the following error. 'Non-manifold edges found for ...'.

I have narrowed the problem to the point that I have removed all the walls except for a vertical plate attaching at the center of a a horizontal plate. I am still getting the problem.

I think that HFSS is confusing whether the attached edge is inner or outer. However, I don't know how to solve this problem.

I will really appreciate your comments.

I have had similar problems
This kind of problem occurs when two objects "touch" each other.refer figure (a)
The remedy is to make objects large enough that they overlap as shown in figure (b).
After that unite or intersect the objects as shown in figure (c).
I hope this will solve your problem
and your program will run happily.:D
tell me how to make a free mirror.

good ,I got the problem too