via structure in hfss

Dear all,

Im having a little trouble with my Via simulation in HFSS V9.2.(see attached file)

Spec of Substrate:
- 4mil thick, Er=3.75
- base copper thickness = 5mil, top copper thickness=1.5mil

Spec of PCB used
- 8mil thick, RO4003, Er=3.55
- base and top copper thickness = 1mil

Do note that i am simulating this by having the substrate sitting on top of the PCB.

The via structure im having is enclosed in a very tight airbox, with the length and width of the substrate. However, it does not make sense to do that(because the structure will ultimately be left in the open), so i tried to increase the width of the airbox. However, the simulation ends with an error message:

"The TMatrix is very close to singular and is not usable for mode calibration. Please revisit the definition of your port terminal lines"

What does this mean? I tried re-defining my wave port lines, but to no avail. Funny thing is, when the airbox is the size of the substrate, it works fine. What do they mean when they say that the TMatrix is very close to singular? If its a problem with my port definition, does it mean that i should use a lumped port instead?

Pls help!.....