Wrong argument for buffer name

[Heime]

Why does the following function give

Lisp error: (wrong-type-argument stringp bfname)

What can I do to fix this problem for the buffer name ?

(defconst buffer-name "BFTEMP")

(defun estring (string &optional bfname)
  "Show STRING in a temporary buffer."

  (or bfname (setq bfname buffer-name))

  (with-output-to-temp-buffer bfname
    (princ string)
    (emacs-lisp-mode)))