The same trend in the band gap bowing is observed for the indirect band gap. Such a trend has never been observed before in any defective perovskite but has been reported for the double perovskite system Cs2AgSb(1-x)BixBr6 and attributed to chemical rather than structural effects. In our case we cannot exclude a synergistic effect due to coexistence of mass disorder and microscopic strain as expected in a two-mode scenario evidenced by Raman data. The present results corroborate the strategy of band gap tuning by Sb/Bi mixing which seems to be general considering the structural difference between defective perovskite and double perovskites and therefore related to the peculiar electronic properties of antimony and bismuth in mixed compositions.