Negative concentration from simulation

Dear NONMEM users, I tried to simulate a new dataset by using a previously published pop pk model. Their model was described by combined error model for residual variability. And after simulation, I have obtained two negative concentrations. I would like to know if there is any proper way to handle those negative concentrations or if there are some codings to prevent gaining negative concentrations. Thanks. Best regards, Nyein Hsu Maung