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Hello all. I am simulating a hot-plate in vacuum that is at a certain boundary held at 400C and at another boundary thermally anchored to room temperature. Since the system is in vacuum, the only thermal losses are by radiation. Attached is the mph file created in COMSOLl 4.4. Upon disabling radiative heat transfer, the simulation seems to work as expected, with the two specified temperature boundaries taking their expected values, with an appropriate gradient between them for the materials in use.

However, when I add "Surface to ambient radiation", setting ambient to room temperature, and when I tick the "Surface-to-surface radiation" box in the Heat Transfer node, COMSOL throws the following error:

Failed to evaluate variable.
- Variable: material.epsilon_rad
- Geometry: 1
- Domain: 1 2 7
Failed to evaluate expression.
- Expression: material.epsilon_rad
Failed to evaluate operator.
- Operator: mean
- Geometry: 1
- Boundary: 15 16
Failed to evaluate expression.
- Expression: mean(material.epsilon_rad)
Failed to evaluate variable.
- Variable: material.epsilon_rad
- Geometry: 1
- Boundary: 15 16 17 23 24 25 28 37 40 64 67
Failed to evaluate variable.
- Variable: comp1.ht.epsilon_rad
- Defined as: material.epsilon_rad
Failed to evaluate expression.
- Expression: d((comp1.ht.epsilon_rad*sigma_const*(comp1.ht.Tamb^4-comp1.ht.T^4)*test(comp1.ht.T))*(dvol),{test@1})


Clearly I need to alter epsilon_rad in the materials of domains 1, 2 and 7 (316L steel) but a velue appears already to exist in the list, so I'm not sure what to change it to. Any ideas?