Detemine expressed b-galactosidase

Question: The question is to detemine whether B-galactosidase is expressed under specified situations:Genotype Glucose only Lactose only Neither glucose nor lactoseI+ P+ O+ Z- / F’ I- P+ Oc Z+ – + +
The answer indictes that the B-galactosidase is expressed whether lactose is present or not. I am confused because I thought that if an I- or Oc (both constituatives) were aIDed to the cell on an F’ then the inducibility is maintained. So I would have thought that the gene would only be expressed if lactose is present (incibility). My question is why is the gene expressed when lactose is not present?