(How) is it possible to cancel a capture?

Especially because of bug #594529 (https://bugs.launchpad.net/sikuli/+bug/594529), I frequently enter capture mode without wanting to. Then I would like to cancel the capture instead of capturing an arbitrary region and having to manually remove it from the script afterwards.

Is there a way to do that? The 'ESC' key pops up as a likely solution but does not work.