flume1.8.0 taildir-source Error: Failed loading positionFile: /usr/op/.flume/taildir_position.json

a1.sources = r1
a1.sinks = k1
a1.channels = c1

## source
a1.sources.r1.type = TAILDIR
a1.sources.r1.positionFile = /tmp/taildir_position.json
a1.sources.r1.filegroups = f1
a1.sources.r1.filegroups.f1 = /tmp/logs/.*log.*
a1.sources.r1.fileHeader = true

# Describe the sink
a1.sinks.k1.type = logger

# Use a channel which buffers events in memory
a1.channels.c1.type = memory
a1.channels.c1.capacity = 1000
a1.channels.c1.transactionCapacity = 100 

# Bind the source and sink to the channel
a1.sources.r1.channels = c1
a1.sinks.k1.channel = c1

2018-02-28 15:12:49,001 (lifecycleSupervisor-1-0) [INFO - org.apache.flume.source.taildir.ReliableTaildirEventReader.<init>(ReliableTaildirEventReader.java:94)] Updating position from position file: /usr/op/.flume/taildir_position.json
2018-02-28 15:12:49,008 (lifecycleSupervisor-1-0) [ERROR - org.apache.flume.source.taildir.ReliableTaildirEventReader.loadPositionFile(ReliableTaildirEventReader.java:146)] Failed loading positionFile: /usr/op/.flume/taildir_position.json
java.io.EOFException: End of input at line 1 column 1
	at com.google.gson.stream.JsonReader.nextNonWhitespace(JsonReader.java:954)
	at com.google.gson.stream.JsonReader.nextValue(JsonReader.java:771)
	at com.google.gson.stream.JsonReader.peek(JsonReader.java:367)
	at com.google.gson.stream.JsonReader.expect(JsonReader.java:337)
	at com.google.gson.stream.JsonReader.beginArray(JsonReader.java:306)
	at org.apache.flume.source.taildir.ReliableTaildirEventReader.loadPositionFile(ReliableTaildirEventReader.java:110)
	at org.apache.flume.source.taildir.ReliableTaildirEventReader.<init>(ReliableTaildirEventReader.java:95)
	at org.apache.flume.source.taildir.ReliableTaildirEventReader.<init>(ReliableTaildirEventReader.java:48)
	at org.apache.flume.source.taildir.ReliableTaildirEventReader$Builder.build(ReliableTaildirEventReader.java:348)
	at org.apache.flume.source.taildir.TaildirSource.start(TaildirSource.java:103)
	at org.apache.flume.source.PollableSourceRunner.start(PollableSourceRunner.java:71)
	at org.apache.flume.lifecycle.LifecycleSupervisor$MonitorRunnable.run(LifecycleSupervisor.java:249)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180)
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
2018-02-28 15:12:49,013 (lifecycleSupervisor-1-0) [INFO - org.apache.flume.instrumentation.MonitoredCounterGroup.register(MonitoredCounterGroup.java:119)] Monitored counter group for type: SOURCE, name: r1: Successfully registered new MBean.
2018-02-28 15:12:49,013 (lifecycleSupervisor-1-0) [INFO - org.apache.flume.instrumentation.MonitoredCounterGroup.start(MonitoredCounterGroup.java:95)] Component type: SOURCE, name: r1 started