Relation between the true UV–optical slope S (≡A₁₅₀₀/A_V) and the stellar mass offset from the true value, derived by fitting the normalised true SFH with the SMC-type dust law under attS_Z_true and attS_Z. The degree of over- or underestimation in M_★ and A_V depends on the steepness of the true normalised attenuation curve relative to the model curve. This behaviour persists even when metallicity is treated as a free parameter, although the fitted values of A_V and M_★ increase slightly.