2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/201118We review the matching conditions for a collapsing anisotropic cylindrical perfect fluid, recently discussed in the literature (2005 {\it Class. Quantum Grav.} {\bf 22} 2407). It is shown that radial pressure vanishes on the surface of the cylinder, contrary to what is asserted in that reference. The origin of this discrepancy is to be found in a mistake made in one step of the calculations. Some comments about the relevance of this result in relation to the momentum of Einstein--Rosen waves are presented.Submitted to Class. Quant. Grav. as a commentGeneral Relativity and Quantum Cosmologytext