I am getting the IIOMetadata object from the JPEG Reader by calling Reader.getImageMetadata(0). From this metadata object I am getting the Node object by calling IIOMetadata.getAsTree() and passing the native metadata format. I called IIOMetadata.mergeTree() and passed the native format name and the Node object obtained from the getAsTree() method. Typically I am merging the same tree with the same metadata object. In this case, mergeTree() is throwing a NPE (for JPEG) which is given below:

java.lang.NullPointerException
at com.sun.imageio.plugins.jpeg.MarkerSegment.getAttributeValue(MarkerSegment.java:131)
at com.sun.imageio.plugins.jpeg.JPEGMetadata.mergeDHTNode(JPEGMetadata.java:1254)
at com.sun.imageio.plugins.jpeg.JPEGMetadata.mergeSequenceSubtree(JPEGMetadata.java:1127)
at com.sun.imageio.plugins.jpeg.JPEGMetadata.mergeNativeTree(JPEGMetadata.java:1092)
at com.sun.imageio.plugins.jpeg.JPEGMetadata.mergeTree(JPEGMetadata.java:1064)
at ReaderImageMetadataTest.<init>(ReaderImageMetadataTest.java:59)
at ReaderImageMetadataTest.main(ReaderImageMetadataTest.java:115)

If mergeTree() does not support merging the same node obtained from getAsTree() it must throw a meaningful exception or IIOInvalidTreeException as specified by the specification. NullPointerExceptions leaves the user clueless on why it is being thrown.

This is reproducible on all platforms right from JDK1.4+.

I have attached a sample code. Execute the sample code as follows.
java ReaderImageMetadataTest <img file>
You will get the above exception in the console.