Word-initial obstruent-liquid clusters, frequent in English (e.g., blue), are prohibited in Korean. Korean learners of English perceptually repair illicit word-initial consonant sequences with an epenthetic vowel [ʊ]. Thus they might perceive blue as b[ʊ]lue, and, at least initially, also represent it lexically as a disyllabic word. We ask whether the sound sequences permitted in one's L1 influence the way L2 words are represented in the mental lexicon. If they do, we predict that in a lexical decision task, Korean learners will accept nonwords containing epenthetic vowels ([bʊˈluː] for blue) as real English words more often than English listeners. These predictions were confirmed: we observed high error rates on test nonwords ([bʊˈluː]) by the Korean participants only, accompanied by few errors on control nonwords ([bɪˈluː]), suggesting that learners’ lexical representations for familiar L2 words can be activated by nonwords that obey their L1 phonotactic grammar.