Invalid heat flux surface goal -- How do I use a component to define heat flux through part of the fluid in Flow Simulation, without affecting the flow?

I'm trying to determine how much heat my heat sink is convecting to the air. I want to measure the heat flux through a particular cross section of space, obviously without affecting the flow in that cross section. I have created a body in that region, and defined a surface goal on desired surface for heat flux. I then disabled the component in "Component Control." When I run the analysis, my surface goals say "Invalid."

I can specify other goals on that surface, such as mass flow rate or temperature of fluid, and I get results as anticipated. It's just heat transfer rate and heat flux that return as "Invalid."

Any ideas?