Firefox Syntax error, unrecognized expression

HTML

<h1>Bug Testing</h1>
<h3>Syntax error, unrecognized expression: # thead tr th:nth-child(2)</h3>

<table id="test">
    <thead>
        <tr>
            <td>
                table head
            </td>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td>
                table cell
            </td>
        </tr>
    </tbody>
</table>

JavaScript

$('#test').find('tbody td').each(function (index) {
    var adid = $(this).find('td:nth-child(2)').text().toLowerCase();
    console.log(adid);
});