Across the Muslim world, Islamic political parties and social organizations have capitalized upon economic grievances to win votes and popular support. But existing research has been unable to disentangle the role of Islamic party ideology from programmatic economic appeals and social services in explaining these parties' popular support. We argue that Islamic party platforms function as informational shortcuts to Muslim voters, and only confer a political advantage when voters are uncertain about parties' economic policies. Using a series of experiments embedded in an original nationwide survey in Indonesia, we find that Islamic parties are systematically more popular than otherwise identical non-Islamic parties only under cases of economic policy uncertainty. When respondents know economic policy platforms, Islamic parties never have an advantage over non-Islamic parties. Our findings demonstrate that Islam's political advantage is real, but critically circumscribed by parties' economic platforms and voters' knowledge of them.