"Element sidebar not allowed as child of element body in this context."

I'm not too sure why this issue is popping up, or if it's really even an issue at all. Any ideas? Line 33, Column 10: Element sidebar not allowed as child of element body in this context.

<body>
<header>
    <a id="logo" href="#"><img src="img/logo.png"></a>
    <nav>
        <ul>
            <li><a href="#">BLOG</a></li>
            <li><a href="#">WORK</a></li>
            <li><a href="#">ABOUT</a></li>
            <li><a href="#">CONTACT</a></li> 
        </ul>
    </nav>
</header>

<div id="container">
    <div id="headline">
        <h1>Headline</h1>
    </div><!-- end headline -->
    <div id="postinfo">
        <p>by me<br />15 April 2012</p>
    </div><!-- end postinfo -->
    <div id="postentry">
        One morning, when Gregor Samsa woke from troubled dreams, he found himself transformed in his bed into a horrible vermin. He lay on his armour-like back, and if he lifted his head a little he could see his brown belly, slightly domed and divided by arches into stiff sections. The bedding was hardly able to cover it and seemed ready to slide off any moment. His many legs, pitifully thin compared with the size of the rest of him, waved about helplessly as he looked. "What's happened to me? " he thought. It wasn't a dream.
    </div><!-- end postentry -->
</div><!-- end container -->

<sidebar>

Solution

There's no element <sidebar> in HTML5 - it was dropped from an early draft several years ago. Consider using <aside> instead.