thermal analysis results

Dear experts, I have done a simple thermal analysis. It is a rectangle with a circular hole in the middle. Initially the whole setup is in room temperature (20 degC). I keep the heat source in the middle and heat it to 100degC. I expect to see a increase in temp from inside to the outside layer. But unfortunately, contrary to my expectation, in the first sub step itself, the temp near the heat source goes below zero. My guess is it is impossible. Please see attached figure. Where could I have gone wrong?
Any advice regarding this analysis is greatly appreciated!!

Thanks
Lax