Getting width of an existing div with jQuery returns null
<p>I have an ASP.NET reportviewer in a page whose width (a div width) I am trying to determine.
I have the test code below. The first alert returns the proper client id. I can see the div in the html source. I can see its width in FireBug. However the second alert returns null. The syntax looks fine. Why is it returning null?</p>

<pre><code>&lt;script type="text/javascript" src="includes/jquery-1.4.2.min.js"&gt;&lt;/script&gt;
&lt;script type="text/javascript" language="javascript"&gt;
alert('&lt;%=rvMain.ClientID %&gt;');
alert( $('#&lt;%=rvMain.ClientID %&gt;').width() );

&lt;/script&gt;
</code></pre>
