Default Calendar Widget not showing

I’m using CSS to control the look of the widget area, but the default calendar widget doesn’t show up, though the title does if you give it one.

You can see what I mean here:

http://blank.wp3.co

All other widgets show up fine.

In my functions file I have the following:

register_sidebar(
	array(
		'name' => 'Sidebar',
		'before_widget' => '',
        'after_widget' => '',
        'before_title' => '<div class="widgettitle">',
        'after_title' => '</div>'
	)
);

And in my style.css file I ahve this:

.widgettitle {
	background: #7f81ff;
	color: #fff;
    width: 230px;
	font-weight: bold;
	text-align: left;
    margin: 0px 10px 5px 10px;
    padding: 5px 5px 5px 5px;
    float: left;
    }

I haven’t made any other changes to that section, and everything else is working so I’m a bit confused.