non-negative constraint to solve time dependent system of PDE using solvepde OR Editing built in functions

I have a systems of time dependent pde's representing population growth and depletion/production of resources. I would like to set the code so that the values of the solutions are not negative (not physical). I am using solvepde, I thought that maybe I could add:

odeoptions=odeset(odeoptions,'NonNegative',1)

in the 72nd line of solveTimeDependent.m but matlab tells me to that my access is denied.

What should I do?

9.0.0.341360 (R2016a)