NullPointerException in background garbage collection

14:55:09,677 ERROR JcrRepository:175 - Error during background garbage collection: null
java.lang.NullPointerException
	at org.modeshape.jcr.SystemContent.systemKey(SystemContent.java:127)
	at org.modeshape.jcr.SystemContent.systemNode(SystemContent.java:174)
	at org.modeshape.jcr.SystemContent.locksKey(SystemContent.java:156)
	at org.modeshape.jcr.SystemContent.mutableLocksNode(SystemContent.java:202)
	at org.modeshape.jcr.SystemContent.cleanUpLocks(SystemContent.java:796)
	at org.modeshape.jcr.JcrRepository$RunningState.cleanUpLocks(JcrRepository.java:1675)
	at org.modeshape.jcr.JcrRepository$LockGarbageCollectionTask.doRun(JcrRepository.java:2001)
	at org.modeshape.jcr.JcrRepository$GarbageCollectionTask.run(JcrRepository.java:1971)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
	at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:351)
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:178)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:178)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:722)

    public NodeKey systemKey() {
        if (systemKey == null) {
            // This is idempotent, so no need to lock
            CachedNode rootNode = system.getNode(system.getRootKey());
            ChildReference systemRef = rootNode.getChildReferences(system).getChild(JcrLexicon.SYSTEM);
            systemKey = systemRef.getKey();
        }
        return systemKey;
    }