Null Pointer Exception in Java 1.5

UGS (ISV) is running into a Null Pointer Exception in a web application deployed in Tomcat 5.1.5 and WebLogic 9.1 using Java 1.5. But the same application works fine in WebLogic 8 and the older version of Tomcat, all of which use Java 1.4. Attached is a small testcase that can show the the Null Pointer Exception.

java.lang.NullPointerException
        at MainXSL.buildKeys()
        at MainXSL.transform()
        at com.sun.org.apache.xalan.internal.xsltc.runtime.AbstractTranslet.transform(AbstractTranslet.java:594)
        at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:640)
        at com.sun.org.apache.xalan.internal.xsltc.trax.TransformerImpl.transform(TransformerImpl.java:279)
        at Main.main(Main.java:46)