The most famous objection to the ontological argument is given in Kant’s dictum that existence is not a real predicate. But it is not obvious how this slogan is supposed to relate to the ontological argument. Some, most notably Alvin Plantinga, have even judged Kant’s dictum to be totally irrelevant to Anselm’s version of the ontological argument. In this paper, I argue, against Plantinga and others, that Kant’s claim is indeed relevant to Anselm’s argument, in the straightforward sense that if the claim is true, then Anselm’s argument is unsound.