[O] [babel] Error when exporting a trivial org file to HTML: Wrong type argument: stringp, (results . "")

[Leo Alekseyev]

Here is what's in my org file:

------------------------8<--------
#+title:   My org file
#+babel: :session *R-babel* :tangle yes

* The problem
** The code
This is going to fail on export:

#+source: test_code
#+BEGIN_SRC R :results output silent :exports none :var foo
  bar <- foo
#+END_SRC

Why does this fail?
#+call: test_code(foo=200)
#+results: test_code(foo=200)
------------------------8<--------

Simple, right?  Yet, when I try to do org-export-as-html, I get:

executing R code block (test_code)...
result is ""
""
executing Emacs-Lisp code block...

(results (quote ""))

Code block produced no output.
org-babel-exp-code: Wrong type argument: stringp, (results . "")

This happens with the latest org-mode from trunk, as well as my
few-months-old version.  What's going on?