Index: post-template.php
===================================================================
--- post-template.php	(revision 12704)
+++ post-template.php	(working copy)
@@ -306,6 +306,9 @@
 function get_post_class( $class = '', $post_id = null ) {
 	$post = get_post($post_id);
 
+	if(is_null($post_id))
+		$post_id = $post->ID;
+	
 	$classes = array();
 
 	if ( empty($post) )