Luke Janicke on "My plugin with 'the_content' filter breaks trying to add excerpt"

This plugin breaks WordPress (blank browser window) if a post has no excerpt. Any idea why?

<?php /*

Plugin Name: Excerpt in Post
Plugin URI:  https://github.com/lukejanicke/excerpt-in-post
Description: Add post excerpts to top of post content output (inc. RSS) wrapped in <code><p class="excerpt">…</p></code>.
Version:     1.0
Author:      Luke Janicke
Author URI:  http://www.lukejanicke.com/
Licence:     GPL2+

*/

function excerpt_in_post( $content ) {

	// Add the excerpt to the top of posts (including in RSS feeds)

	$excerpt = get_the_excerpt();

	if ( $excerpt ) $content = sprintf( '<p class="excerpt">%s</p>%s', $excerpt, $content );

	return $content;

}
add_filter( "the_content", "excerpt_in_post", 20 );
add_filter( "the_content_feed", "excerpt_in_post", 20 );

?>