  It was proposed that the narrow $P_c(4450)$ structure observed by the LHCb
Collaboration in the reaction $\Lambda_b\to J/\psi p K$ might be due to a
triangle singularity around the $\chi_{c1}$--proton threshold at 4.45~GeV. We
discuss the occurrence of a similar triangle singularity in the $J/\psi p$
invariant mass distribution for the decay $\Lambda_b\to J/\psi p \pi$, which
could explain the bump around 4.45~GeV in the data. More precise measurements
of this process would provide valuable information towards an understanding of
the $P_c$ structures.
