IndexOutOfBoundsException from FieldsReader after problem reading the index

There is a situation where there is an IOException reading from Hits, and then the next time you get a NullPointerException instead of an IOException.

Example stack traces:

java.io.IOException: The specified network name is no longer available
at java.io.RandomAccessFile.readBytes(Native Method)
at java.io.RandomAccessFile.read(RandomAccessFile.java:322)
at org.apache.lucene.store.FSIndexInput.readInternal(FSDirectory.java:536)
at org.apache.lucene.store.BufferedIndexInput.readBytes(BufferedIndexInput.java:74)
at org.apache.lucene.index.CompoundFileReader$CSIndexInput.readInternal(CompoundFileReader.java:220)
at org.apache.lucene.store.BufferedIndexInput.refill(BufferedIndexInput.java:93)
at org.apache.lucene.store.BufferedIndexInput.readByte(BufferedIndexInput.java:34)
at org.apache.lucene.store.IndexInput.readVInt(IndexInput.java:57)
at org.apache.lucene.index.FieldsReader.doc(FieldsReader.java:88)
at org.apache.lucene.index.SegmentReader.document(SegmentReader.java:344)
at org.apache.lucene.index.IndexReader.document(IndexReader.java:368)
at org.apache.lucene.search.IndexSearcher.doc(IndexSearcher.java:84)
at org.apache.lucene.search.Hits.doc(Hits.java:104)

That error is fine. The problem is the next call to doc generates:

java.lang.NullPointerException
at org.apache.lucene.index.FieldsReader.getIndexType(FieldsReader.java:280)
at org.apache.lucene.index.FieldsReader.addField(FieldsReader.java:216)
at org.apache.lucene.index.FieldsReader.doc(FieldsReader.java:101)
at org.apache.lucene.index.SegmentReader.document(SegmentReader.java:344)
at org.apache.lucene.index.IndexReader.document(IndexReader.java:368)
at org.apache.lucene.search.IndexSearcher.doc(IndexSearcher.java:84)
at org.apache.lucene.search.Hits.doc(Hits.java:104)

Presumably FieldsReader is caching partially-initialised data somewhere. I would normally expect the exact same IOException to be thrown for subsequent calls to the method.