In interactive models of speech production, wordforms that are related to a target form are co-activated during lexical planning, and co-activated wordforms can leave phonetic traces on the target. This mechanism has been proposed to account for phonetic similarities among morphologically related wordforms. We test this hypothesis in a Javanese verb paradigm. In Javanese, one class of verbs is inflected by nasalizing an initial voiceless obstruent: one form of each word begins with a nasal, while its otherwise identical relative begins with a voiceless obstruent. We predict that if morphologically related forms are co-activated during production, the nasal-initial forms of these words should show phonetic traces of their obstruent-initial forms, as compared to nasal-initial wordforms that do not alternate. Twenty-seven native Javanese speakers produced matched pairs of alternating and non-alternating wordforms. Based on an acoustic analysis of nasal resonance and closure duration, we present good evidence against the original hypothesis: We find that the alternating nasals are phonetically identical to the non-alternating ones on both measures. We argue that interactive effects during lexical planning do not offer the best account for morphologically conditioned phonetic similarities. We discuss an alternative involving competition between phonotactic constraints and word-specific phonological structures.