Cannot deselect all options in a custom alphanumeric checkbox field

See this example on the demo site:
http://drupal.demo.civicrm.org/civicrm/contact/view?reset=1&action=browse&cid=102&gid=2&selectedChild=custom_2

If you attempt to un-check all of the options in Test 3 and then save, your changes will not be saved. If you choose a different set of options, and then save, it works as expected. There seems to be a bug in the form postProcess step.