The URL above should look like

<message name="thismessagedoesntexist">&lt;thismessagedoesntexist&gt;</message>

rather than

<message name="thismessagedoesntexist">&amp;lt;thismessagedoesntexist&amp;gt;</message>

(in the HTML source, this is &amp;amp;lt; which should be &amp;lt;)

Version: 1.13.x
Severity: enhancement
URL: http://en.wikipedia.org/w/api.php?action=query&meta=allmessages&ammessages=thismessagedoesntexist