Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 12704)
+++ wp-includes/post-template.php	(working copy)
@@ -304,6 +304,9 @@
  * @return array Array of classes.
  */
 function get_post_class( $class = '', $post_id = null ) {
+	if ( is_null($post_id) )
+		$post_id = get_the_ID();
+
 	$post = get_post($post_id);
 
 	$classes = array();