Index: wp-content/themes/twentythirteen/functions.php
===================================================================
--- wp-content/themes/twentythirteen/functions.php	(revision 23468)
+++ wp-content/themes/twentythirteen/functions.php	(working copy)
@@ -409,7 +409,7 @@
 	}
 
 	// Post author
-	if ( 'post' == get_post_type() ) {
+	if ( 'post' == get_post_type() && is_multi_author() ) {
 		printf( '<span class="author vcard"><a class="url fn n" href="%1$s" title="%2$s" rel="author">%3$s</a></span>',
 			esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
 			esc_attr( sprintf( __( 'View all posts by %s', 'twentythirteen' ), get_the_author() ) ),