Lensing boost factor b_L for the values of P₀ = P(μ > 2, z_s > 6) considered in this Letter, P₀ = 0.92, P₀ = 0.20 and P₀ = 0.04. The horizontal line indicates the value of P₀ that makes the undetected population equal in number to the detected population. We assume a flux-limited survey with a flux limit of z_AB = 22.0 (Wang et al. 2017).