Can't access a variable's children in extension element

Hi all,

I'm having an issue with evaluating an XPath expression in an extension
element. It works fine for almost all cases I've tried so far, but if I have
a variable in my stylesheet which has child nodes under it, I'm having
difficulty when I try to resolve any of the children more than once. So for
a variable $blahVar which resolves to this element:

<entry code="blah">
   <firstamount>12345</firstamount>
   <secondamount>67890</secondamount>
</entry>

... I can pass my element something like <tst:myElement
value="$blahVar/firstamount" /> and have it work on '12345', however after
accessing the variable for the first time any subsequent accesses fail. If I
put that same call in again, or substitute $blahVar/secondamount then the
XPath variable always resolves to the empty string.

This is obviously possible - I have xsl:value-of doing the exact same
lookups with the same XPath strings right alongside and getting the correct
results. I've been checking through the xalan-j source this morning to see
if I could see what I'm missing, but I'm still quite new to xalan-j and
nothing has jumped out at me so far.

The code I'm using is as follows:

    protected static String xpathLookup(XSLProcessorContext context,
ElemExtensionCall call, String exp) throws TransformerException
    {
        String nodeText = "";

        if ((exp != null && exp.length() != 0))
        {
            TransformerImpl transformer = context.getTransformer();
            XPathContext xpContext = transformer.getXPathContext();

            try
            {
                xpContext.pushNamespaceContext(call);
                int current = xpContext.getCurrentNode();
                xpContext.pushCurrentNodeAndExpression(current,current);

                XPath dynamicXPath = new XPath(
                    exp,
                    xpContext.getSAXLocator(),
                    xpContext.getNamespaceContext(),
                    XPath.SELECT,
                    transformer.getErrorListener());

                Expression expression = dynamicXPath.getExpression();

                XObject xobj1 = expression.execute(xpContext);
                xpContext.popCurrentNodeAndExpression();
                xpContext.popNamespaceContext();
                nodeText = xobj1.toString();
            }
            catch(TransformerException te) {}
        }

        return nodeText;
    }

I know this has to be something really simple that I'm missing, any help is
appreciated.

Thanks,
Peter Carberry.