One of the most significant recent developments in sexual offence policy has been the proliferation of affirmative consent policies, which prohibit sexual activity with someone who has not acted in a way that clearly expresses her consent. I argue that if this prohibition has a proportionate sanction, such a policy can be defended on the grounds that we have a duty of due diligence investigate other people's willingness to participate in sexual activity. However, I pose a challenge for the view that communicative behaviour is necessary for valid consent from a moral point of view.