Problem with interpolation function and variable

Hi !

I'm a beginner in COMSOL 4.2a and I'm trying to simulate a model using the Microwave Heating interface.
I have to consider the temperature dependance for the thermal conductivity and the electrical conductivity.
For now, I just create an interpolation function, named "func1", which has only one argument (temperature)
and giving the corresponding thermal conductivity.

I also define a variable "var1" to create a link with the function, and I use it as a value of the thermal conductivity.

But after simulation, I get this message :

Failed to evaluate variable Jacobian
- Variable : T
- Geometry : 1
- Domain : 1

The domain 1 is a material using the variable "var1" as thermal conductivity.

Does anyone know how to solve this problem ? Thanks in advance.

Nicolas.