Buying random votes is as hard as buying no-votes

In voting systems where a mark in a fixed position may mean a vote for Alice on a ballot, and a vote for Bob on another ballot, an attacker may coerce voters to put their mark at a certain position, enforcing effectively a random vote. This attack is meaningful if the voting system allows to take receipts with them and/or posts them to a bulletin board. The coercer may also ask for a blank receipt. We analyze this kind of attack and prove that it requires the same effort as a comparable attack would require against any voting system, even one without receipts.