Does replace-match needs a literal TO-STRING?

[Thorsten Jolitz]

Hi List, 

I try to use 
  
#+BEGIN_SRC emacs-lisp
(when (search-forward FROM-STRING nil t)
    (replace-match TO-STRING nil t))
#+END_SRC

in a temp-buffer, and it works when TO-STRING is literal like "foo", but
not when its a function that builds and then returns the string "foo". 

Is that a known restriction or am I doing something wrong?

-- 
cheers,
Thorsten