Some applications of Nash bargaining in the antitrust analysis of reverse payment settlements estimate the bargaining power parameter as the split of the surplus in the actual settlement with alleged reverse payments. This estimated parameter is then used as the bargaining power parameter in the but-for world where reverse payments are prohibited. We demonstrate that this approach is incorrect. Indeed, in the but-for world there could theoretically be an alternative “no payment” settlement where reverse payments are prohibited. However, the prohibition on reverse payments disproportionately affects the shape of the bargaining set in favor of the patent holder. [---]