我在hadoop集群上运行nutch，爬网的数据存储在cassandra集群中。运行nutch作业时，出现以下错误：

java.lang.NullPointerException
    at org.apache.avro.util.Utf8.<init>(Utf8.java:38)
    at org.apache.nutch.crawl.GeneratorReducer.setup(GeneratorReducer.java:100)
    at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:174)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:649)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:418)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:255)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1190)
    at org.apache.hadoop.mapred.Child.main(Child.java:249)

我一开始是这样做的：

$HADOOP_HOME/bin/hadoop jar /nutch/apache-nutch-2.2.1.job org.apache.nutch.crawl.Crawl urls -dir crawl -depth 3 -topN 5