I am currently doing post layout extraction simulation on Spectre. I am using Columbus RF tool for my layout extraction and Spectre to run my simulation. After post layout extraction, I am facing problem at Spectre when I changed alway the process corner to negative sigma. It will indicated the following messages:
" Error found by spectre in 'pfet': '+19' duirng hierachy flattening. I0.+19.djdb_area of pfet: Value of 'area' should be greater than or equal to zero"

I have try running the simulation at 0 sigma process corner and +3 sigma process corner, the simulation is able to run but I always face the above problem when I switch to -3 sigma corner. What causes this problem? Can anyone help me to solve this issue? Thank you