[O] org-babel-execute:dot -- why isn't this working?

[Matt Price]

I'm trying to draw a very simple, silly graph using dot. I'm following the code here:
http://irreal.org/blog/?p=2866

and here:
http://orgmode.org/worg/org-tutorials/org-dot-diagrams.html

I have tables like this (the real ones are substantially longer):

#+name: students-graph
| a | b |
| a | m |
| a | f |
| b | t |

#+name: students-table
| *node* | *label*   | *shape* | *fillcolor* |
|--------+-----------+---------+-------------|
| a      | Omar      | ellipse | green       |
| b      | Hindia    | ellipse | orange      |
| c      | Yuvrai    | ellipse | purple      |

I can generate the diagram I'm looking for with this code, mostly stolen from another :
#+name: make-dot
#+BEGIN_SRC emacs-lisp :var table=students-table graph=students-graph :results output :exports none
  (mapcar #'(lambda (x)
              (princ (format "%s [label=\"%s\" shape=%s style=\"filled\" fillcolor=\"%s\"];\n"
                             (car x) (second x) (nth 2 x) (nth 3 x) ))) table)
  (mapcar #'(lambda (x)
              (princ (format "%s -- %s;\n"
                             (first x) (second x)))) graph)


#+END_SRC