XPath read root element returns null

Given an XML file like:

<source>
   <element value="a">
   <element value="b">
</source>

I'm trying to read the root element ("source") of the XML using Java and XPath:

public String parseExpression(Document doc)  {
   NodeList nodeList = (NodeList) xPath.compile("/").evaluate(
                    doc, XPathConstants.NODESET);
   return nodeList.item(0).getFirstChild().getNodeValue();
}

However it returns null. Why ?

Thanks