Unlike the singulars of other declensions, which formed their vocatives as a rectus casus identical to the nominative, the second declension masculine vocatives of nouns ending in -us declined, sometimes ending in -e, sometimes in -i.
ibidem, page 146:
He regards the letters i, u, and v infilius, fatuus,andservusas vowels, and hence treats them as part of the termination. What is more important, the author attempts to address ambiguities that might arise in the forms of words containing such semi-vowels by regularizing them according to the rule of analogy, and treating their vocatives as examples of the rectus casus, as is the case in the other declensions. This allows him to present the vocatives“o puer”and“o vir,”which normally are rectus casus, on the same level as“o fatuus”and“o servus,”which are clearly not standard.
