NPE exporting a log in LogStateMachine

2019-09-04 16:32:21,638 WARN  server.LogStateMachine (LogStateMachine.java:processArchiveLog(686)) - Exception while processing archival request for LogName['testLogExport']
java.lang.NullPointerException
        at org.apache.ratis.logservice.server.LogStateMachine.processArchiveLog(LogStateMachine.java:619)
        at org.apache.ratis.logservice.server.LogStateMachine.access$6(LogStateMachine.java:599)
        at org.apache.ratis.logservice.server.LogStateMachine$7.run(LogStateMachine.java:310)
        at org.apache.ratis.statemachine.impl.BaseStateMachine.recordTime(BaseStateMachine.java:210)
        at org.apache.ratis.logservice.server.LogStateMachine.query(LogStateMachine.java:308)
        at org.apache.ratis.server.impl.RaftServerImpl.submitClientRequestAsync(RaftServerImpl.java:553)
        at org.apache.ratis.server.impl.RaftServerProxy.lambda$submitClientRequestAsync$7(RaftServerProxy.java:333)
        at org.apache.ratis.server.impl.RaftServerProxy.lambda$null$5(RaftServerProxy.java:328)
        at org.apache.ratis.util.JavaUtils.callAsUnchecked(JavaUtils.java:109)
        at org.apache.ratis.server.impl.RaftServerProxy.lambda$submitRequest$6(RaftServerProxy.java:328)
        at java.util.concurrent.CompletableFuture.uniComposeStage(CompletableFuture.java:981)
        at java.util.concurrent.CompletableFuture.thenCompose(CompletableFuture.java:2124)
        at org.apache.ratis.server.impl.RaftServerProxy.submitRequest(RaftServerProxy.java:327)
        at org.apache.ratis.server.impl.RaftServerProxy.submitClientRequestAsync(RaftServerProxy.java:333)
        at org.apache.ratis.grpc.client.GrpcClientProtocolService$RequestStreamObserver.processClientRequest(GrpcClientProtocolService.java:220)
        at org.apache.ratis.grpc.client.GrpcClientProtocolService$UnorderedRequestStreamObserver.processClientRequest(GrpcClientProtocolService.java:276)
        at org.apache.ratis.grpc.client.GrpcClientProtocolService$RequestStreamObserver.onNext(GrpcClientProtocolService.java:240)
        at org.apache.ratis.grpc.client.GrpcClientProtocolService$RequestStreamObserver.onNext(GrpcClientProtocolService.java:168)
        at org.apache.ratis.thirdparty.io.grpc.stub.ServerCalls$StreamingServerCallHandler$StreamingServerCallListener.onMessage(ServerCalls.java:248)
        at org.apache.ratis.thirdparty.io.grpc.internal.ServerCallImpl$ServerStreamListenerImpl.messagesAvailable(ServerCallImpl.java:263)
        at org.apache.ratis.thirdparty.io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1MessagesAvailable.runInContext(ServerImpl.java:686)
        at org.apache.ratis.thirdparty.io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37)
        at org.apache.ratis.thirdparty.io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:123)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)