Why doesn't this work? StAX support in JAXP xml validation API problem

Alright I give up. . .on paper (meaning according to the API documentation)
what I'm trying to do should work.
I want to hand a javax.xml.transform.stax.StAXSource to a Validator object's
validate() method to have it validate some xml input against an XML Schema.
The schema loads and compiles fine. The XML input file loads fine. I
believe that both the schema file and the xml data file are in fact valid
and well-formed. But as soon as I call the validate() method on the
Validator object passing in the StAXSource object I created, I get a
NullPointerException in a stack trace as follows:

ERROR: ''
Exception in thread "main" java.lang.NullPointerException
at
com.sun.org.apache.xerces.internal.jaxp.validation.StAXValidatorHelper.validate(StAXValidatorHelper.java:95)
at
com.sun.org.apache.xerces.internal.jaxp.validation.ValidatorImpl.validate(ValidatorImpl.java:114)
at javax.xml.validation.Validator.validate(Validator.java:127)
at test.StaxExample.methodA(StaxExample.java:43)
at test.StaxExample.main(StaxExample.java:22)

This happens using Java 6 SDK update 6. I also tried downloading the latest
nightly build JAXP 1.4 reference implementation from the official JAXP
website and set the java.endorsed.dirs System Property to override the JAXP
classes that come standard with JDK 1.6 but to no avail. Got the same NPE.
I even tried compiling and running my code under JDK 1.5 where the classes
in question do not even exist as part of the standard JDK so I knew for sure
it was loading the classes from the desired JAXP nightly build jars just as
a sanity check but to no avail. Same error. So I can only conclude at this
point that either I am misunderstanding something and doing something wrong
(more likely) or there is a bug in the JDK (less likely but not unheard of).
If in fact I am doing something that was not intended to be done but that is
still allowed by the API, at the very least I'd think the code should throw
an UnsupportedOperationException or an IllegalArgumentException or something
like that rather than just an NPE. Anyway the code is as follows (I
apologize in advance for the formatting - or lack thereof as the case may
be - if whatever medium you are reading this message on has boogered it all
up):

<code>
package test;

import java.io.File;
import java.io.FileInputStream;

import javax.xml.XMLConstants;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamReader;
import javax.xml.transform.Source;
import javax.xml.transform.stax.StAXSource;
import javax.xml.validation.Schema;
import javax.xml.validation.SchemaFactory;
import javax.xml.validation.Validator;

public class StaxExample {

 public static void main(String[] args) throws Exception {
  if (args.length != 2) {
   System.out.println("Usage : StaxExample <xmlFile> <schemaFile>");
  } else {
   methodA(args[0], args[1]);
  }
 }

 private static void methodA(String xmlFile, String xsdFile)
 throws Exception {
  XMLInputFactory xmlif = XMLInputFactory.newInstance();
  XMLStreamReader staxReader = xmlif.createXMLStreamReader(new
FileInputStream(xmlFile));

  SchemaFactory schemaFactory = SchemaFactory
  .newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
  Schema schemaGrammar = schemaFactory.newSchema(new File(xsdFile));

  Validator schemaValidator = schemaGrammar.newValidator();

  Source staxSrc = new StAXSource(staxReader);
  schemaValidator.validate(staxSrc);

  while (staxReader.hasNext()) {
   int eventType = staxReader.next();
   System.out.println("Event of type: " + eventType);
  }
 }
}
</code>

Thanks in advance to anyone who can offer some advice, pointers, ridicule,
public humiliation, etc. . .
I'd really like to get to the bottom of this be it because this is a genuine
bug or if it is because of something I'm doing wrong due to my own
ignorance.
And thanks to 'hille' who's code I ripped off the java.net forums for my
example code above and who also had the same problem (about a year ago now
from the date on his forum post) but never got an answer as to why the
validation framework doesn't work with a StAXSource object:
http://forums.java.net/jive/thread.jspa?messageID=223721