[Nutch2.3]调用generate的时候出现java.util.NoSuchElementException异常

kandy@ubuntu:~/workspace/nutch/runtime/local$  ./bin/nutch inject urls/url.txt -crawlId 6vhao
InjectorJob: starting at 2016-02-22 22:27:00
InjectorJob: Injecting urlDir: urls/url.txt
InjectorJob: Using class org.apache.gora.memory.store.MemStore as the Gora storage class.
InjectorJob: total number of urls rejected by filters: 0
InjectorJob: total number of urls injected after normalization and filtering: 1
Injector: finished at 2016-02-22 22:27:02, elapsed: 00:00:01
kandy@ubuntu:~/workspace/nutch/runtime/local$ ./bin/nutch generate -crawlId 6vhao
GeneratorJob: starting at 2016-02-22 22:27:26
GeneratorJob: Selecting best-scoring urls due for fetch.
GeneratorJob: starting
GeneratorJob: filtering: true
GeneratorJob: normalizing: true
java.util.NoSuchElementException
at java.util.TreeMap.key(TreeMap.java:1323)
at java.util.TreeMap.firstKey(TreeMap.java:290)
at org.apache.gora.memory.store.MemStore.execute(MemStore.java:125)
at org.apache.gora.query.impl.QueryBase.execute(QueryBase.java:73)
at org.apache.gora.mapreduce.GoraRecordReader.executeQuery(GoraRecordReader.java:68)
at org.apache.gora.mapreduce.GoraRecordReader.nextKeyValue(GoraRecordReader.java:110)
at org.apache.hadoop.mapred.MapTask$NewTrackingRecordReader.nextKeyValue(MapTask.java:531)
at org.apache.hadoop.mapreduce.MapContext.nextKeyValue(MapContext.java:67)
at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:144)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:764)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:364)
at org.apache.hadoop.mapred.LocalJobRunner$Job$MapTaskRunnable.run(LocalJobRunner.java:223)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
GeneratorJob: finished at 2016-02-22 22:27:28, time elapsed: 00:00:01
GeneratorJob: generated batch id: 1456151246-1968731436 containing 0 URLs