Invocation of com.xpn.xwiki.doc.XWikiDocument#getPreviousVersion produce a NullPointerException

Invocation of com.xpn.xwiki.doc.XWikiDocument#getPreviousVersion produce a NullPointerException in (XWikiDocument.java:5224)
when triggered from an org.xwiki.observation.EventListener

Here is the stack trace :

2009-09-01 14:17:21,084 http://88.191.79.179:8080/xwiki/bin/view/Main/ http://88.191.79.179:8080/xwiki/bin/view/Main/?null ERROR rnal.DefaultObservationManager - Fail to send event [org.xwiki.observation.event.DocumentUpdateEvent@160ce61] to listener [com.xpn.xwiki.annotation.internal.maintainment.JavaDiffBasedAnnotationMaintainer@1b7b32c]
java.lang.NullPointerException
at com.xpn.xwiki.doc.XWikiDocument.getPreviousVersion(XWikiDocument.java:5224)
at com.xpn.xwiki.annotation.internal.maintainment.AbstractAnnotationMaintainer.onEvent(AbstractAnnotationMaintainer.java:175)
at org.xwiki.observation.internal.DefaultObservationManager.notify(DefaultObservationManager.java:250)
at org.xwiki.observation.internal.DefaultObservationManager.notify(DefaultObservationManager.java:224)
at com.xpn.xwiki.XWiki.saveDocument(XWiki.java:1330)
at com.xpn.xwiki.XWiki.saveDocument(XWiki.java:1265)
at com.xpn.xwiki.XWiki.saveDocument(XWiki.java:1260)