Deus / meus etc
It is fairly common in Latin for the nominative to
appear where we expect a vocative, especially in elevated
language e.g. audi tu, populus Albanus! or from Horace:
almae filius Maiae ("O son of dear Maia").
Deus shows no vocative other than "deus" before
Tertullian's dee, but meus has the vocative mi, which is fairly regular, but not
consistent - the vocative meus also occurs.
Puer has the expected vocative puere in early
Latin.
Peter