model of error, linearisation, additive error, and VPC

Hello NONMEM Users, When I tried to print log-scaled VPC, there was an error message about negative values. It can be caused by an additive error and/or linearization of the error model when NONMEM transforms Y = F*DEXP(ERR(1)*SD1) into Y = F+ F*ERR(1)*SD1. The error is inflated at small concentrations and removing the additive term is not an option in my case. Is there an easy way to solve it? It can be something like Y = F**GAMMA*DEXP(ERR(1)*SD1) or Y = F+ F*ERR(1)*SD1 + F**GAMMA*ERR(2)*SD2, where GAMMA<1. Thanks, Pavel