Using document.evaluate in Greasemonkey

Question

Using Greasemonkey, I want to automatically click on this element:

<input class="autostar {split:2}" type="radio" value="5.0" name="rating-1" style="display:none" f="5">

I have written this, but it doesn't work:

var result = document.evaluate("//input[@type='radio' and @name='rating-1 and @value='5.0']",
document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null);

window.onload = function () {
    result.click();
}