LhsEval getInvB_(typename std::enable_if<!HasMember_invB<FluidState>::value,
                                         const FluidState&>::type fluidState,
                 unsigned phaseIdx,
                 unsigned pvtRegionIdx)
{
    const auto& rho = fluidState.density(phaseIdx);
    const auto& Xsolvent =
        fluidState.massFraction(phaseIdx, FluidSystem::solventComponentIndex(phaseIdx));

    return
        Opm::decay<LhsEval>(rho)
        *Opm::decay<LhsEval>(Xsolvent)
        /FluidSystem::referenceDensity(phaseIdx, pvtRegionIdx);
}