While most European languages obligatorily mark number on nouns, many other languages have an opposition between a bare general number form and the plural (Corbett 2000). Such 'Optional Number Languages' are Korean (Kim 2005), Balinese (Arka & Dalrymple 2017), Buriat (Bylinina & Podobryaev t.a.), and Bangla (Biswas 2013). We present novel evidence from Korean and Bangla that plural marking on indefinites is a positive polarity item. We derive this pattern from the assumptions that bare stems in optional number languages are ambiguous between a singular form and a number-neutral bare form, and that the plural is unmarked (e.g. Sauerland et al. 2005). We then argue that pragmatic competition derives the positive polarity effect because the bare stem and the plural form are logically equivalent.