DERIVESTsuite: jacobian of a MATLAB defined function

I am using 'jacobianest' function of the MATLAB file exchange file 'DERIVEstsuite'. My function is defined only for positive values of the variables. So, whenever jacobianest tries to evaluate the function at non-positive values of the variable, I get an error because my function cannot be evaluated. How can I compute Jacobian of my function which is defined only for strictly positive values of the variables.