Can't update relationship by name using API after changing name in the web interface

If a relationship name is changed in the web interface, using 'relationship_type' in civicrm_relationship_create() no longer works. The error is 'Invalid Relationship Type' even though it does exist.

Change the relationship 'Child of/Parent of' to 'Kid of/Guardian of'. Assuming child is id 1 and parent is id 2, call the API:
$params = array('contact_id_a' => 1, 'contact_id_b' => 2, 'relationship_type' => 'Kid of', 'is_active' => 1);
$result =& civicrm_relationship_create($params);
print_r($result);

Will return:
Array
(
[is_error] => 1
[error_message] => Invalid Relationship Type
)