Unable to evaluate speed and pressure in a solid?

Hi everyone,

I'm doing a simulation of a boiler using both the heat transfer module and the CFD module.
The simulation always stops because the solver is not able to compute the value of pressure and speed in a given point of the domain. But when I checked the position of this point, I figured out that it is inside a solid.

I already checked and I'm sure that I defined the fluid domains and solid domains correctly. So, why is Comsol trying to compute the pressure and the speed inside the solid?

Thanks in advance for any help you may provide,
- Davide Ivaldi