We performed a search simultaneously for the HeH+ J = 1-0 line (νrest= 2010.183 873 GHz) and for the CH 2Π3/2(F2)J = 3/2-2Π1/2(F2)J = 1/2 lines (νrest≈ 2006.8 and 2010.8 GHz) towards one of the highest redshift quasars known, SDSS J114816.64+525150.3 (z = 6.4189). No clearly visible line was detected after obtaining an rms noise level of ̃0.4 mK (̃3 mJy) in 16-MHz (18 km s-1) channels. At a level of 2.9σ, however, there is a tentative emission feature shifted by about 100 km s-1 from the expected frequency of the HeH+ line. This shift is well within the width of the line profiles for CO and C+. The putative feature is about four times narrower than the previously detected CO and C+ lines. The difference in velocities as well as in the linewidths could be explained by quite different conditions required for the formation and excitation of HeH+ with respect to CO and C+. The HeH+ emission, if real, could probably arise in the dense ionized gas of this quasi-stellar object. The velocity-integrated flux in this tentative feature is 0.62 ± 0.21 Jy km s-1 which corresponds to a total luminosity of L(HeH+) ≈ 7.1 × 108 L☉. As long as there is no independent confirmation, these values should be considered rather as upper limits.