problem with babel and dot

[Eric S Fraga <e.fraga@ucl.ac.uk>]

Hello,

I have the following emacs lisp code to build up a dot input for a
dependency graph:

#+begin_src org
  ,#+name: graph-from-tables
  ,#+header: :var options="" :var nodes='() graph='()
  ,#+BEGIN_SRC emacs-lisp 
    (org-babel-execute:dot
     (concat
          "digraph {\n"
          options "\n"   ;; "//rankdir=LR;\n" ;; remove comment characters '//' for horizontal layout; add for vertical layout
          (mapconcat
           (lambda (x)
             (format "%s [label=\"%s\" shape=%s style=\"filled\" fillcolor=\"%s\"]"
                             (car x)
                             (nth 1 x)
                             (if (string= "" (nth 2 x)) "box" (nth 2 x))
                             (if (string= "" (nth 3 x)) "none" (nth 3 x))
                             )) nodes "\n")
          "\n"
          (mapconcat
           (lambda (x)
             (format "%s -> %s [taillabel=\"%s\"]"
                             (car x) (nth 1 x) (nth 2 x))) graph "\n")
          "}\n") params)
  ,#+END_SRC
#+end_src

This used to work but was last used probably a year ago or more.

I invoke this function elsewhere in the document with a call like this:

#+begin_src org
  ,#+call: graph-from-tables[:file dependency-graph.pdf](options="rankdir=LR;",nodes=subtasks-table[2:-1],graph=dependency-table[2:-1]) :results file :exports results
#+end_src

Two tables are used to describe the dependency graph and the information
passed to the function above works just fine.  The input file for dot
also looks perfect (and if I run dot on it, I get the graph I
want).  However, the result of the call here is a file,
dependency-graph.pdf, which has 3 bytes: "nil".

I have no idea how to get this working again.  ob-dot.el provides no
clues to me unfortunately.

Any pointers please?

Many thanks,
eric

-- 
: Eric S Fraga (0xFFFCF67D), Emacs 25.0.90.1, Org release_8.3.3-565-g4f499f