Index: wp-includes/category-template.php
===================================================================
--- wp-includes/category-template.php	(revision 25605)
+++ wp-includes/category-template.php	(working copy)
@@ -136,8 +136,13 @@
 function get_the_category_by_ID( $cat_ID ) {
 	$cat_ID = (int) $cat_ID;
 	$category = get_term( $cat_ID, 'category' );
+	
 	if ( is_wp_error( $category ) )
 		return $category;
+		
+	if ( null === $category ) 
+		return new WP_Error( 'invalid_category', __( 'Invalid category' ) );
+
 	return $category->name;
 }
 
Index: wp-includes/category.php
===================================================================
--- wp-includes/category.php	(revision 25605)
+++ wp-includes/category.php	(working copy)
@@ -76,7 +76,7 @@
  * @param int|object $category Category ID or Category row object
  * @param string $output Optional. Constant OBJECT, ARRAY_A, or ARRAY_N
  * @param string $filter Optional. Default is raw or no WordPress defined filter will applied.
- * @return mixed Category data in type defined by $output parameter.
+ * @return object|array|WP_Error|null Category data in type defined by $output parameter.
  */
 function get_category( $category, $output = OBJECT, $filter = 'raw' ) {
 	$category = get_term( $category, 'category', $output, $filter );