How to find the index of a row in a table using jquery

I'm trying to find the index of a row in a table. I'm trying to use the following code, but I seem to get an index of -1.

$(document).ready(function()
{
    $("tr").click(function (){
        var index = $("table").index($(this));
        $("span").text("That was row index #" + index);
    });
});

With html that looks like this;

<table>
<tbody>
    <tr><td>click</td></tr>
    <tr><td>click</td></tr>
    <tr><td>click</td></tr>
    <tr><td>click</td></tr>
</tbody>

Thanks