Why is غُلَامِي genitive in مَرَرْتُ بِالْعَصَا وَغُلَامِي?

Noun Case Class 7 — Maqṣūr and Nouns with Possessive Yāʾ

• Simply because it ends in yāʾ
• Because its conjunct partner is genitive
• Because it is a subject
• Because it is indeclinable