16:17:28,276 SEVERE [Jenkins] Failed Loading plugin checkstyle

After upgrading to the newest version of Jenkins the checkstyle plugin will no longer load.
This plugin loaded fine in Jenkins 1.409.3

This same error appears in the logs for
Jenkins ver. 1.439-SNAPSHOT (rc-11/08/2011 02:30 GMT-jenkins)
and
ver. 1.438
and
ver. 1.437

The Checkstyle plugin version 3.21

If I roll back to Jenkins 1.409.3 the errors go away.

Attached is the relevant part of the logs.