Citizens in John Rawls's well-ordered society face an assurance dilemma. They wish to act justly only if they are reasonably sure their fellow citizens will also act justly. According to Rawls, this assurance problem is solved through public reasoning. This paper argues that public reason cannot serve this function. It begins by arguing that one kind of incompleteness public reason faces that most Rawlsians grant is ubiquitous but unproblematic from a normative standpoint is problematic from an assurance perspective: it makes it possible for citizens to argue for policy conclusions that are favored by their private interests, rather than justice. In response, perhaps the thing to do is structure deliberative democratic institutions such that citizens will always be incentivized to use public reasons to only argue for conclusions they believe are favored by justice. The paper proves that this is impossible by extending the Gibbard–Satterthwaite theorem.