The intuition that knowledge requires the satisfaction of some sort of anti-luck condition is widely shared. I examine the claim that modal robustness is sufficient for satisfying this condition: for a true belief to be non-luckily true, it is sufficient that this belief is safe and sensitive. I argue that this claim is false by arguing that, at least when it comes to beliefs in necessary truths, satisfying the anti-luck condition requires satisfying a non-modal condition. I also advance a plausible candidate for this condition and argue for the implausibility of mathematical Platonism on this basis.