Why is there kasrah on Aḥmad in «مَرَرْتُ بِأَحْمَدِكُمْ»?

Restoring Declension: Indefinite Names, Al- and Iḍāfah

• The noun is the subject
• The noun is the first member of iḍāfah
• It has accusative tanwīn
• It is a sound feminine plural