The word rātri optionally takes mum augment before a word formed by kṛt affix.,
As रात्रिचरः or रात्रिंचरः, रात्रिमटः or रात्र्यटः ॥ This is an aprapta-vibhasha. The augment is compulsory before a krit-affix having an indicatory ख, As, रात्रिंमन्यः ॥,