Failed to find a solution. There are 1 void equations

Hello,

I'm running a flow in a pipe and I'm using the Conjugate heat transfer physics. My model converges and gives me results when the fluid in the pipe is water. As soon as I change the fluid to Carbon Dioxide, I get the following error:

Failed to find a solution.
Singular matrix.

There are 1 void equations (empty rows in matrix) for the variable comp1.nitf.Pmfinl1.
at coordinates: (0,0,0), ...
There are 28 void equations (empty rows in matrix) for the variable comp1.p.
at coordinates: (0,-1.82291,0.0448905), (0,-1.8415,-6.245e-017), (0,-1.778,0.0635), (0,-1.82291,-0.0448896), (0,-1.73308,0.0448825), ...
There was an error message from the linear solver.
The relative residual (11) is greater than the relative tolerance.
Returned solution is not converged.
- Feature: Stationary Solver 1 (sol2/s1)


Can anyone tell me why when changing fluids my model wouldn't converge even though everything else is still the same? And what is the variable comp1.nitf.Pmfinl1?

Thank you