Let X be a nonempty, topologically complete metric space with no isolated points. We show that there exists a closed upper porous set (in a strong sense) F ⊂ X which is not σ-lower porous (in a weak sense). More precisely, we show that there exists a closed (g1)-shell porous set F ⊂ X which is not σ-(g2)-lower porous, where g1 and g2 are arbitrary admissible functions.