Asserting public domain causes the CC Zero license icon to be shown, but clicking on that icon takes the user to the following URL:

http://www.opendatacommons.org/licenses/pddl/summary/

that is not the CC Zero license; it's the ODC Public Domain Dedication and License (PDDL).

The correct URL for CC Zero appears to be:

http://creativecommons.org/publicdomain/zero/1.0/