ditaa diagram not as expected

[Richard Lewis <richardlewis@fastmail.co.uk>]

Hi there,

I'm preparing a document in which I discuss the CSS box model and
wanted to include a diagram, this diagram:

#+begin_ditaa boxmodel.png -S
+---=---------------------------------------+
|           margin                          |
|  +-------------------------------------+  |
|  |        border                       |  |
|  |  +----=--------------------------+  |  |
|  |  |     padding                   |  |  |
|  |  |  +----=--------------------+  |  |  |
|  |  |  |  content                |  |  |  |
|  |  |  |                         |  |  |  |
|  |  |  |                         |  |  |  |
|  |  |  +----=--------------------+  |  |  |
|  |  |                               |  |  |
|  |  +----=--------------------------+  |  |
|  |                                     |  |
|  +-------------------------------------+  |
|                                           |
+-------=-----------------------------------+
#+end_ditaa

However, the diagram that org/ditaa creates is missing the third box
(counting from the outside inwards), the box between "border" and
"padding". I tried removing the "=" (which would make the line solid),
but the box still did not appear. I've tried this snippet in an org
document by itself, and I've tried putting the diagram in a file by
itself and running ditaa directly. Still no middle box.

I realise this may be a ditaa rather than an org-mode problem, but can
anyone suggest anything? Particularly, does anyone else get the same
effect?

org-mode version: 7.8.11; ditaa version: 0.9; Emacs version: 24.1.1.

Thanks,
Richard