We show that the molecular dynamics simulation scheme for geometrically constrained systems at constant pressure proposed by Melchionna and Ciccotti [J. Chem. Phys. 106, 195 (1997)] is not consistent with the desired isobaric ensemble. The extended system method necessitates to impose the standard geometrical constraints for the physical momenta explicitly. A solution to this problem has been recently presented by Kneller and Mülders [Phys. Rev. E, 54, 6825 (1996)]