NodeList keeps returning null when evaluating XPath

Question

I am trying to evaluate my xpath with the following code in Java, but nodeList returns null - why does this happen? My XPath seems to be evaluating pretty well here: https://stackoverflow.com/a/46833473/436493.

Java

String message = "<AMUpdate><AMMessageType>AMPRICES</AMMessageType><amprice:AMPrices xmlns:amprice="http://www/am.com/am/dto/price"><Price><Currency>GBP</Currency><Country>LU</Country><BusinessLine>AMSL</BusinessLine></Price></amprice:AMPrices></AMUpdate>"
XPath xpath = XPathFactory.newInstance().newXPath();
InputSource source = new InputSource(new StringReader(message));
NodeList nodeList ((NodeList) xpath.evaluate("/AMUpdate/amprice:AMPrices/Price/*", source, XPathConstants.NODESET)).item(0).getChildNodes();

XML

<AMUpdate><AMMessageType>AMPRICES</AMMessageType><amprice:AMPrices xmlns:amprice="http://www/am.com/am/dto/price"><Price><Currency>GBP</Currency><Country>LU</Country><BusinessLine>AMSL</BusinessLine></Price></amprice:AMPrices></AMUpdate>

Update #2

I've now tried the following but nodelist still returns null:

    String message = "<AMUpdate><AMMessageType>AMPRICES</AMMessageType><amprice:AMPrices xmlns:amprice="http://www/am.com/am/dto/price"><Price><Currency>GBP</Currency><Country>LU</Country><BusinessLine>AMSL</BusinessLine></Price></amprice:AMPrices></AMUpdate>"
    XPath xpath = XPathFactory.newInstance().newXPath();

xpath.setNamespaceContext(new NamespaceContext() {
    public String getNamespaceURI(String prefix) {
        if (prefix == null) throw new NullPointerException("Null prefix");
        else if ("am".equals(prefix)) return "http://www/am.com/am/dto/price";
        else if ("xml".equals(prefix)) return XMLConstants.XML_NS_URI;
        return XMLConstants.NULL_NS_URI;
    }

    // This method isn't necessary for XPath processing.
    public String getPrefix(String uri) {
        throw new UnsupportedOperationException();
    }

    // This method isn't necessary for XPath processing either.
    public Iterator getPrefixes(String uri) {
        throw new UnsupportedOperationException();
    }
});

    InputSource source = new InputSource(new StringReader(message));
    NodeList nodeList ((NodeList) xpath.evaluate("/AMUpdate/amprice:AMPrices/Price/*", source, XPathConstants.NODESET)).item(0).getChildNodes();