How do free-trade areas affect the tariffs that member countries impose against non-members? There is no consensus in the literature regarding this important question. In this study, we use a political-economy model of endogenous protection to show that if individual preferences exhibit loss aversion or if the government's objective is characterized by diminishing marginal political support, a free-trade area can lead member countries to increase their external tariffs and thus act as a stumbling block to unilateral liberalization. We also argue that the stumbling block result is more likely to arise under loss aversion than under diminishing marginal political support and confirm this using a simulation. Finally, we show that the stumbling block effect can also take place under multilateral liberalization. Our results highlight a new type of mechanism through which preferential trade agreements may affect external tariffs.