Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 22289)
+++ wp-includes/post-template.php	(working copy)
@@ -299,13 +299,11 @@
 /**
  * Retrieve the classes for the post div as an array.
  *
- * The class names are add are many. If the post is a sticky, then the 'sticky'
- * class name. The class 'hentry' is always added to each post. For each
- * category, the class will be added with 'category-' with category slug is
- * added. The tags are the same way as the categories with 'tag-' before the tag
- * slug. All classes are passed through the filter, 'post_class' with the list
- * of classes, followed by $class parameter value, with the post ID as the last
- * parameter.
+ * There are many class names added. 'sticky' if the post is sticky.
+ * 'hentry' for all. 'category-[slug id]' for categories, same for tags.
+ * All classes are passed through the filter 'post_class' with the list
+ * of classes, which is then followed by the $class argument value, 
+ * with the $post_id as the last argument.
  *
  * @since 2.7.0
  *
@@ -319,7 +317,7 @@
 	$classes = array();
 
 	if ( empty($post) )
-		return $classes;
+		return apply_filters('post_class', $classes, explode( ' ', $class ), null);
 
 	$classes[] = 'post-' . $post->ID;
 	if ( ! is_admin() )