The capacity to act in accordance with one’s morality (broadly construed) is constitutive of moral agency. This capacity can be undermined—in whole or in part—by for instance, hypnosis, addiction, or obsessive-compulsion. Another way this capacity can be undermined is through poor moral reasoning. Moral irrationality can frustrate one’s capacity to act in accordance with one’s morality and in turn, stunt one’s moral agency. In a similar respect, improving moral rationality can strengthen this capacity and enhance moral agency. The empirical research program on (non-moral) cognitive debiasing inspires developing techniques to improve our moral rational capacities—i.e., moral debiasing. Yet, moral debiasing presupposes moral biases—that is, systematic moral errors. So, what are moral errors exactly? The pertinent kind is subjective moral errors. Ultimately, A’s φ-ing is a subjective moral error insofar as φ-ing deviates from A’s genuine morality per instrumental subjective moral rationality (ISMR)—i.e., insofar as φ-ing frustrates A+’s morally-relevant ends, wherein A+ is a counterfactual idealization of A upon whom is bestowed those endowments that A considers authoritative under ordinary optimal conditions. The provision of an in-principle standard of subjective moral error lays important theoretical groundwork for future empirical inquiry into subjective moral debiasing.