Shortcode widget issue

Hi there – i Have a simple shortcode for making a sidenote ..

Although it works when I look at the post – it doesnt seem to allow the publish screen on the backend to work properly – the page just goes blank on publish/update .. what am i doing wrong?!
here’s the plugin code..

add_shortcode('bigfoot',function($vars,$highlight){
	// ----------------- defaults -----------------------//
	$vars = shortcode_atts( // overide itself with the following!
		array(
			'comment'=>'you arent sayin nothin',
			'highlight' => !empty($highlight)? $highlight : 'i am empty' // if not empty then use the input OR default!
		),$vars
	);
	// ----------------- defaults -----------------------//

	extract($vars); // php method to extract array elements as instant variables! so no more $vars['linkurl']

	return "<span class='underline'>$highlight</span><div class='sidenote'>$comment</div>"; 

});

usage in post content would be:

my text etc [bigfoot comment=’here is my comment’]and here is special text[/bigfoot]and the main content carries on …..

thanks!!!