1.2.0 categories.rename always fails with "invalid user"

If anyone can see what I'm doing wrong, I'd appreciate it.

I'm trying to execute the categories.rename method as follows:

http://api.smugmug.com/hack/json/1.2.0/?method=smugmug.categories.rename
&APIKey=<API KEY>
&SessionID=<SESSION ID>
&CategoryID=<CATEGORY ID>
&Name=<NEW CATEGORY NAME>

And I'm always getting back an error-code 4 (Invalid user) reply from the server.

My test case is written in the following fashion:

1. Login (withPassword)
2. Create Category
3. **Rename Category
4. Delete Category

Steps 1, 2 and 4 all work (using the same API Key and Session ID, which is usually the culprit for invalid user) so I think something must be amiss with how rename is implemented or maybe I'm doing something wrong? :scratch