  As part of a study BSM corrections to leptonic decays of the $B_c$ meson,
Tran et al. (arXiv:1801.06927) use the covariant confining quark model (CCQM)
to estimate the matrix element of the pseudo-scalar curent between the vacuum
and the $B_c$ meson. We note that this matrix element can be determined using
existing lattice QCD results.
