  We 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.
