The error-related negativity (ERN) is a negative deflection in the event-related brain potential associated with error processing. A recent theory holds that the ERN is elicited by the impact of a reward prediction error signal carried by the mesencephalic dopamine system on anterior cingulate cortex. The theory predicts that larger ERNs should be elicited by unexpected unfavorable outcomes than by expected unfavorable outcomes. We tested the theory in an experiment in which the frequency of occurrence of reward was varied by condition, reasoning that the system that produces the ERN would come to expect non-reward when rewards were infrequent. Consistent with the theory, we found that larger ERNs were elicited by unexpected absences of reward.