  The NA61/SHINE collaboration measured inclusive cross sections of pi+ and pi-
meson production in the interactions of 31 GeV/c proton with carbon nuclei at
small emission angles (0 - 420 mrad). The collaboration presented also
predictions of Monte Carlo models - FLUKA, VENUS and UrQMD, in a comparison
with the data. The worst description of the data was observed for UrQMD model
results. In the present paper it is shown that the drawback of the UrQMD model
is connected with an inaccurate treatment of low mass string fragmentation. The
strings appeared at a diffraction of target nucleons. A simple patch is
proposed to overcome the problem.
