This paper discusses the highly irregular suffix allomorphy observed in Nuer and identifies patterns with tonal polarity. In oblique case constructions, nouns can be inflected through suffixation or non-concatenative forms. I argue that the choice of these two allomorphs are connected with the underlying stem tones, which is revealed by tonal polarity: Toneless nouns tend to undergo non-concatenative inflection, and their stems are subject to tonal polarity, while L-toned nouns tend to get a suffix with a polar tone. In an OT analysis with an input allomorph set, I show that the choice of allomorphs is phonological-conditioned for these nouns. The main idea is that non-concatenative forms arise through so-called bidirectional defectiveness when both the stem and affix material are defective.