This paper provides an analysis of the typologically unusual patterns of case assignment in Tlapanec. The proposal challenges some idiosyncratic aspects of former descriptive analyses (i.e. zero-marked ergative and a novel case 'pegative') and provides a theoretically and typologically more adequate analysis instead. The theoretical approach relies on the framework of Distributed Morphology (DM). According to this theory the output of syntax can be impoverished before the lexical insertion of concrete vocabulary items takes place. This framework is modified in the account proposed here, insofar as the step of impoverishment of morphosyntactic features is designed as an optimality theoretic competition. I will show that all of the constraints that are crucial for this competition emerge from independently established prominence scales by a modified version of harmonic alignment. Thereby the process of impoverishment is given a typologically/conceptually more plausible base.