For the short i and u final in a nominal-stem, a guṇa is substituted in the Vocative Case singular.,
As हे अग्ने, हे वायो, हे पटो ॥ But there is no guna in हे कुमारि, हे ब्रह्मबन्धु, because ई and ऊ were shortened specifically, and to substitute guna for them would make their shortening a useless operation. Moreover, had guna been intended in the case of these nadi words, the sutra would have been अम्बार्थानां ह्रस्वः, २ नदीह्रस्वयोर्गुणः ॥,