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We consider ambiguity-averse agents with maxmin expected utility (Gilboa and Schmeidler, 1989) in the Bayesian persuasion model by Kamenica and Gentzkow (2011). With no prior ambiguity, the sender choose to send an ambiguous signal with multiple likelihood distributions. We provide a version of Bayes plausibility when ambiguous signals are allowed. We illustrate how the "revelation principle" a la Kamenica and Gentzkow (2011) might fail.  Compared to the classic Bayesian persuasion model, the sender may do strictly better by sending an ambiguous signal. This provides justification for how vagueness may emerge endogenously in persuasion, when the sender has full commitment.