NPE in TransactionImpl.checkForAckCollision()

Occurred during the following build while running the TrieIT:

https://travis-ci.org/mikewalch/Accismus/builds/28625179

java.lang.NullPointerException
at accismus.impl.TransactionImpl.checkForAckCollision(TransactionImpl.java:504)
at accismus.impl.TransactionImpl.preCommit(TransactionImpl.java:367)
at accismus.impl.TransactionImpl.preCommit(TransactionImpl.java:312)
at accismus.impl.TransactionImpl.commit(TransactionImpl.java:636)
at accismus.impl.Worker.processUpdates(Worker.java:175)
at accismus.impl.WorkerTask.run(WorkerTask.java:66)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at java.lang.Thread.run(Thread.java:744)