java.io.IOException: Cannot join values on null key. Did you specify a key column that exists?

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Cannot join values on null key. Did you specify a key column that exists?
java.io.IOException: Cannot join values on null key. Did you specify a key column that exists?
at org.apache.sqoop.mapreduce.MergeMapperBase.processRecord(MergeMapperBase.java:79)
at org.apache.sqoop.mapreduce.MergeTextMapper.map(MergeTextMapper.java:58)
at org.apache.sqoop.mapreduce.MergeTextMapper.map(MergeTextMapper.java:34)
at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:146)
at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:787)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:341)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1657)

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