Index: wp-includes/post-template.php
===================================================================
--- wp-includes/post-template.php	(revision 22378)
+++ wp-includes/post-template.php	(working copy)
@@ -318,8 +318,13 @@
 
 	$classes = array();
 
+	if ( !empty($class) ) {
+		if ( !is_array( $class ) )
+			$classes = preg_split('#\s+#', $class);
+	}
+
 	if ( empty($post) )
-		return $classes;
+		return array_map( 'esc_attr', $classes );
 
 	$classes[] = 'post-' . $post->ID;
 	if ( ! is_admin() )
@@ -366,12 +371,6 @@
 		}
 	}
 
-	if ( !empty($class) ) {
-		if ( !is_array( $class ) )
-			$class = preg_split('#\s+#', $class);
-		$classes = array_merge($classes, $class);
-	}
-
 	$classes = array_map('esc_attr', $classes);
 
 	return apply_filters('post_class', $classes, $class, $post->ID);