Assempde restrict solution space

Hello, im trying to compute an elliptic equation with Neumann boundary conditions. I'm using the assempde.m function and I have matlab 2014 (so I cannot use Matlab's pde model container).

As it is known if we have a solution u for this problem, then for any constant c, u+c is also a solution. So it is needed to impose some extra condition (could be mean zero on the domain or on the boundary), else the matrix is not invertible. I'm not finding any simple way to impose a condition, and when I'm solving matlab is complaining about having to invert an ill conditioned matrix, so I guess it does not does it automatically.

How could I solve this issue?

Thanks.