Weak Constraint thermal flux

Hi every body

I am using Weak form to model strongly coupled thermal-structural equations.My model consists of two layers.
I used Weak constraint to ensure that my thermal flux will be continuous but
Comsol gives me an error that "Expression must be linear in test function"..and i dont know how to solve it.
What does it mean?.Do I have to change a shape function?if yes,how can I do that

I used (q22-q2)*test(u3), where u3 is dependent variable for temperature and q variable for heat flux of each layer.

Coud you please help me?

Thank you