Can't integrate over a cross-section!

I'd like to be able to integrate over an arbitrary cross section of my model. I'm trying to integrate the poynting vector over a hole in a cavity after doing modal analysis. I'm coupling a cavity to a piece of microstrip that sits above. I'm running into some difficulty, though. My first attempt was just just going to derived values->surface integration, and then attempting to click on the hole itself... but there's really no surface defined IN the hole, so I can't select it. So instead, I go and create a new work plane, and create a circular surface inside the hole, and figure I could integrate over this. No go... the circular surface I made disappears when I switch over to define the surface integral! As a last resort, I create a 2D slice under Data Sets of the relevant cross section, and figure I'll just integrate over that, since the power flux I'm interested in is only nonzero in the region of interest... so it works out fine. Unfortunately, when I hit integrate, it just gives NaN! Yaaargh! I tried just picking the max value over the defined 2D slice, and this works just fine... Any ideas what might be going on?

The poynting vector I have defined as real(-Ey*conj(emw.Hx) + Ex*conj(emw.Hy))... I can plut this on a 2D plot surface just fine... comsol just won't integrate over it! Also, I'd love to know why the surface defined inside the hole disappears!