CompositeRecordReader: key and values can be in uninitialized state if files being joined have no records

I am using org.apache.hadoop.mapred.join.CompositeInputFormat to do an outer-join across a number of SequenceFiles. This works fine in most circumstances, but I get NullPointerExceptions/uninitialized data (where Writable#readFields() has not been called) when some of the files being joined have no records in them.