How can I get which radio is selected via jQuery under IE7 and IE8?

Base on this question radio selected

I use this way

<input type="radio" class="fp" value="q" name="fp" checked>
fp = $('input[name=fp]:checked').val();
alert(fp);

To get the value of select radio box. Please see this fiddle

But it is not worked under IE7 and IE8. Always got "Object doesn't support this property or method" . Any suggestions?