pp-to-string, defun and "()"

[yzhh]

The following code is intended for produce the string of a defun, 
(pp-to-string
           `(defun recorded-commands ()
              "Commands recorded during the last 'recording commands' cycle."
              (interactive)
              ,@(reclisp-post-process reclisp-recorded-commands)))

It produces 
(defun recorded-commands nil "Commands recorded during the last 'recording
commands' cycle."
  (interactive)
...

The "()" is translated to "nil" which does not look good, and the doc string
is put in the same line place. Any suggesting?

-- 
   regards,
yzhh