Using XPath to get an elmement in SVG

I am trying to get an element in an SVG file using XPath. I have tried the following code, but singleNodeValue is null. The doc seems to be correct, so I guess either evaluate() arguments or the XPath is wrong, but I cannot find anything wrong. Why doesn’t it work?

JavaScript

fetch('test.svg')
.then(response => response.text())
.then(data=>{
    const parser = new DOMParser();
    const doc = parser.parseFromString(data, "text/xml");
    console.log(doc);
    const res = doc.evaluate("//symbol[@label='square']", doc, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null);
    console.log(res.singleNodeValue);
})

SVG

<?xml version="1.0" encoding="UTF-8" standalone="no"?>

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">

    <symbol label ="square">
        <rect y="5" x="5" width="90" height="90" stroke-width="5" stroke="#f00" fill="#f00" fill-opacity="0.5" />
    </symbol>

</svg>