java.lang.NullPointerException

[Lakshmi Anantharaman]

ultWebApp_lukara)] Root cause of ServletException
java.lang.NullPointerException
        at
org.apache.fop.fo.FOTreeBuilder.startElement(FOTreeBuilder.java:218)
        at
org.apache.fop.tools.DocumentReader.parse(DocumentReader.java:444)
        at org.apache.fop.apps.Driver.render(Driver.java:482)

Can anybody provide some input on this .
I am doinga Driver.render(Document)
where Driver is takingthe xsl-fo as input Source !

Thanks
LAkshmi