A donation may have ambiguous costs or ambiguous benefits. Behavior in a laboratory experiment suggests that individuals use this ambiguity strategically as a moral wiggle room to act less generously without feeling guilty. Such excuse-driven behavior is more pronounced when the costs of a donation - rather than its benefits - are ambiguous. However, the importance of excuse-driven behavior is comparable under ambiguity and under risk. Individuals exploit any type of uncertainty as an excuse not to give, regardless of the nature of this uncertainty.