Although segmentally reconstructible as *-ъ in late Proto-Slavic, the ending of the gen. pl. had prosodic effects that suggest it was phonetically longer than other final jers. The explanation proposed here is that the jer of the gen. pl. was formerly nasalized (*-ъ̨) and hence redundantly longer than its oral counterpart. Nasalized *-ъ̨ was the regular reflex, via the intermediate stage *-ūn, of Balto-Slavic non-acute *-ōn < PIE *-oHom.