  We present an update on the calculation of $\bar{B}\to D^\ast \ell \bar{\nu}$
semileptonic form factor at zero recoil using the Oktay-Kronfeld bottom and
charm quarks on $N_f=2+1+1$ flavor HISQ ensembles generated by the MILC
collaboration. Preliminary results are given for two ensembles with $a\approx
0.12$ and $0.09$ fm and $M_\pi\approx 310$ MeV. Calculations have been done
with a number of valence quark masses, and the dependence of the form factor on
them is investigated on the $a\approx 0.12$ fm ensemble. The excited state is
controlled by using multistate fits to the three-point correlators measured at
4--6 source-sink separations.
