Error while solving Joule heating: mesh-case 0 there are 665 void equations (empty rows in matrix) for the variable mod1.V

Hi,

I tried to simulate joule heating in a copper cylindrical wire with an cylindrical air-void in it. But I am getting following error:

Failed to find a solution.
Singular matrix.

For mesh-case 0 there are 665 void equations (empty rows in matrix) for the variable mod1.V
at coordinates: (1.2,-0.7,2), (1.18384,-0.573893,2), (1.13639,-0.455939,2), (1.13771,-0.941685,2), (1.18418,-0.824792,2), ...
and similarly for the degrees of freedom (empty columns in matrix)
Returned solution is not converged.

I saw similar posts, but couldn't simulate my model still; the model is attached herewith.

Observation: If I take the difference of these two cylinders and remove the air material (i.e. the final geometry would have just the copper wire with a groove), COMSOL is able to solve it. But, I need to simulate the air void too. In one case, I would also like to have the small cylindrical air-void completely inside the bigger cylinder.

Any help is greatly appreciated.

Thanks,
Kapil