This paper addresses the compositionality of the French modal verb devoir (‘must’) in past conditional sentences, focusing on the intriguing fact that such sentences tend to trigger counterfactual interpretations. This is puzzling in particular because neither the verb nor the tense would achieve this interpretation alone. We suggest that this is due to a conflict between evidential, modal and temporal features, from which a meaning of wrong past expectations, hence counterfactuality, emerges pragmatically.