Can't remove Amazon DRM on two books from public domain

[waryde]

Hi,
I have used the Calibre Plugin successfully on a lot of Amazon books, but I found two where it doesn't work.
You can test it, because the books are free! (I need to remove DRM because I have my Kindle in an German Amazon account and some books are only available in the US-Store.)
I have Kindle for PC 1.4.1 installed and an account at Amazon.com.
I can remove DRM on all other books I tried so far.
The two books are: (as mentioned, they are free, so please try)
The Art of War
and
The Time Machine

If I try to import them on the console I get:

MOBI header version = 5, length = 232
Extra Data Flags = 1
Crypto Type is:  2
Running file type plugin K4PC, K4Mac, Kindle Mobi and Topaz DeDRM failed with traceback:
Traceback (most recent call last):
  File "site-packages\calibre\customize\ui.py", line 148, in _run_filetype_plugins
  File "calibre_plugins.dummy1.__init__", line 341, in run
Exception: K4MobiDeDRM plugin could not decode the file
Backing up metadata
Notifying calibre of the change

Can anyone else remove the DRM on them? Or is there a bug in the plugin?