In this paper, I discuss variation in case marking on subjects of non-finite relative clauses (RCs) in Hill Mari (Finno-Ugric, Uralic) which relativize on a constituent other than subject, as well as φ-feature agreement on two different heads: the nominal and the participial head. I use these novel data to argue in favor of configurational case assignment as opposed to case assignment by a particular head. Adopting the configurational theory of case, I argue that NOM appears on clause-internal subjects as the unmarked case in the clausal domain. GEN, on the other hand, is the realization of unmarked case in the nominal domain, where two subject positions, external and internal, are available. Two dialects of Hill Mari each realize one of these options, which can be distinguished by a number of diagnostics.