وَمِنْهَا: «أَمْسِ» بِالْكَسْرِ عِنْدَ أَهْلِ الْحِجَازِ.
Translation: Amsi is built on kasra in the Hijazi usage.
The source finally notes the Hijazi building of amsi on kasra. The ending is a fixed building vowel; it does not automatically indicate the genitive case.