illegalargumentexception in logs

Any idea what might be causing this, and how I might be able to resolve it?

2015-02-20 09:42:33,008 ajp-bio-8009-exec-25662 ERROR bwell1 582x2309803x3 14egng8 72.37.244.140 /rest/issueNav/1/issueTable [common.error.jersey.ThrowableExceptionMapper] Uncaught exception thrown by REST service: No enum constant com.xpandit.raven.requirement.a.UNCOVERED;2013
java.lang.IllegalArgumentException: No enum constant com.xpandit.raven.requirement.a.UNCOVERED;2013
at java.lang.Enum.valueOf(Unknown Source)
at com.xpandit.raven.requirement.a.valueOf(Unknown Source)
at com.xpandit.raven.customfield.searchers.a.a(Unknown Source)
at com.xpandit.raven.customfield.searchers.a.compare(Unknown Source)
at com.atlassian.jira.issue.search.parameters.lucene.sort.MappedSortComparator$InternalFieldComparator.compare(MappedSortComparator.java:85)