NullPointerException IndexRowGenerator.getExecutionFactory

We have had a couple of customers which have experienced the crash show below in the derby.index-stat-thread.
We can't recreate the issue locally, but what may be the cause of this? Is there anything in particular that may be provoking this?
 
java.lang.NullPointerExceptionat org.apache.derby.iapi.sql.dictionary.IndexRowGenerator.getExecutionFactory(Unknown Source)at org.apache.derby.iapi.sql.dictionary.IndexRowGenerator.getIndexRowTemplate(Unknown Source)at org.apache.derby.iapi.sql.dictionary.IndexRowGenerator.getNullIndexRow(Unknown Source)at org.apache.derby.impl.services.daemon.IndexStatisticsDaemonImpl.updateIndexStatsMinion(Unknown Source)at org.apache.derby.impl.services.daemon.IndexStatisticsDaemonImpl.generateStatistics(Unknown Source)at org.apache.derby.impl.services.daemon.IndexStatisticsDaemonImpl.processingLoop(Unknown Source)at org.apache.derby.impl.services.daemon.IndexStatisticsDaemonImpl.run(Unknown Source)at java.lang.Thread.run(Thread.java:748)