Index: src/wp-includes/category-template.php
===================================================================
--- src/wp-includes/category-template.php	(revision 25675)
+++ src/wp-includes/category-template.php	(working copy)
@@ -136,9 +136,11 @@
 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;
-	return $category->name;
+
+	return ( $category ) ? $category->name : '';
 }
 
 /**
Index: src/wp-includes/category.php
===================================================================
--- src/wp-includes/category.php	(revision 25675)
+++ src/wp-includes/category.php	(working copy)
@@ -76,10 +76,11 @@
  * @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. WP_Error if $category is empty, null if it does not exist.
  */
 function get_category( $category, $output = OBJECT, $filter = 'raw' ) {
 	$category = get_term( $category, 'category', $output, $filter );
+
 	if ( is_wp_error( $category ) )
 		return $category;
 
@@ -284,7 +285,7 @@
  * @param int|object $tag
  * @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 object|array Return type based on $output value.
+ * @return object|array|WP_Error|null Tag data in type defined by $output parameter. WP_Error if $tag is empty, null if it does not exist.
  */
 function get_tag( $tag, $output = OBJECT, $filter = 'raw' ) {
 	return get_term( $tag, 'post_tag', $output, $filter );