Missing Argument Warning

Hi, I have the following code in my functions.php to show a lightbox on the Simple Paypal Shopping Cart Plugin

function make_wpspsc_thumbs_lightbox_previews( $thumbnail, $atts ) {

	// Code Credit: http://stackoverflow.com/questions/19323324/how-to-get-image-src-attribute-value-from-php-string

	$doc = new DOMDocument();

	// Let's make sure we don't execute this code on the wrong elements & blow up the site
	if ( ! is_object( $doc ) ) {
		return $thumbnail;
	}

	libxml_use_internal_errors(true);
	$doc->loadHTML( $thumbnail );
	$xpath = new DOMXPath($doc);
	$imgs = $xpath->query("//img");
	for ($i=0; $i < $imgs->length; $i++) {
    		$img = $imgs->item($i);
    		$src = $img->getAttribute("src");
	}
	// Now let's use the image src we just got & rel="lightbox" because that's what Responsive Lightbox from dFactory uses
	$new_thumbnail = '<a href="'.$src.'" rel="lightbox">'.$thumbnail.'</a>';

	return $new_thumbnail;
}
add_filter( 'wspsc_product_box_thumbnail_code', 'make_wpspsc_thumbs_lightbox_previews' );

When I add something the the cart I get the warning:

Warning: Missing argument 2 for make_wpspsc_thumbs_lightbox_previews()

On line 16 which is this:

function make_wpspsc_thumbs_lightbox_previews( $thumbnail, $atts ) {

Can anyone see what is causing the problem please?