diff --git a/wp-admin/admin-db.php b/wp-admin/admin-db.php
index 0e2b388cac..3d26d8eeb9 100644
--- a/wp-admin/admin-db.php
+++ b/wp-admin/admin-db.php
@@ -108,7 +108,7 @@ function wp_insert_category($catarr) {
 	$category_description = apply_filters('pre_category_description', $category_description);
 
 	$category_parent = (int) $category_parent;
-	if ( empty($category_parent) || !get_category( $category_parent ) || $category_parent == $cat_ID )
+	if ( empty($category_parent) || !get_category( $category_parent ) || ($cat_ID && _cat_is_ancestor_of($cat_ID, $category_parent) ) )
 		$category_parent = 0;
 
 	if ( isset($posts_private) )
diff --git a/wp-includes/category.php b/wp-includes/category.php
index 7b3e7cae33..690c196ecf 100644
--- a/wp-includes/category.php
+++ b/wp-includes/category.php
@@ -225,6 +225,21 @@ function get_cat_name($cat_id) {
 	return $category->cat_name;
 }
 
+function cat_is_ancestor_of($cat1, $cat2) { 
+	if ( is_int($cat1) ) 
+		$cat1 = & get_category($cat1); 
+	if ( is_int($cat2) ) 
+		$cat2 = & get_category($cat2); 
+
+	if ( !$cat1->cat_ID || !$cat2->category_parent ) 
+		return false; 
+
+	if ( $cat2->category_parent == $cat1->cat_ID ) 
+		return true; 
+
+	return cat_is_ancestor_of($cat1, get_category($cat2->parent_category)); 
+} 
+
 //
 // Private
 //