Index: wp-includes/category-template.php
===================================================================
--- wp-includes/category-template.php	(revision 23762)
+++ wp-includes/category-template.php	(working copy)
@@ -72,6 +72,11 @@
  * @return array
  */
 function get_the_category( $id = false ) {
+	if( !$id ) {
+                global $post;
+                $id = $post->ID;
+        }
+
 	$categories = get_the_terms( $id, 'category' );
 	if ( ! $categories || is_wp_error( $categories ) )
 		$categories = array();