Journal article: Day and Milgrom (Int J Game Theory 36:393-407, 2008) argue that package auctions that select the seller's minimum revenue in the Core are revenue-monotone. We show that no bidder-optimal Core-selecting auction can satisfy revenue-monotonicity for general preferences when there are at least three goods for sale, while the property holds for any bidder-optimal Core-selecting auction in environments with only two goods or if the characteristic function is submodular.