Cannot cast java.lang.String to org.joda.time.DateTime

Stack trace

java.lang.ClassCastException: Cannot cast java.lang.String to 
org.joda.time.DateTime
	at java.lang.Class.cast(Class.java:3369)[?:1.8.0_111]
	at org.graylog2.plugin.Message.getFieldAs(Message.java:380)[graylog.jar:?]
	at org.graylog2.plugin.Message.getTimestamp(Message.java:178)[graylog.jar:?]
	at com.graylog.splunk.output.senders.TCPSender.send(TCPSender.java:151)[?:?]
	at com.graylog.splunk.output.SplunkOutput.write(SplunkOutput.java:87)[?:?]
	at org.graylog2.buffers.processors.OutputBufferProcessor$1.run(OutputBufferProcessor.java:189)[graylog.jar:?]
	at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:176)[graylog.jar:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)[?:1.8.0_111]
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)[?:1.8.0_111]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)[?:1.8.0_111]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)[?:1.8.0_111]
	at java.lang.Thread.run(Thread.java:745)[?:1.8.0_111]