In a buffer with text before the first heading, I get an error using org-map-entries in this function:

(defun org-export-all (backend)
  "Export all subtrees that are *not* tagged with :noexport: to
separate files.

Note that subtrees must have the :EXPORT_FILE_NAME: property set
to a unique value for this to work properly."
  (interactive "sEnter backend: ")
  (let ((fn (cond ((equal backend "html") 'org-html-export-to-html)
                  ((equal backend "latex") 'org-latex-export-to-latex)
                  ((equal backend "pdf") 'org-latex-export-to-pdf))))
    (save-excursion
      (set-mark (point-min))
      (goto-char (point-max))
      (org-map-entries (lambda () (funcall fn nil t)) "-noexport" 'region-start-level))))

The error is :
Debugger entered--Lisp error: (error "Before first headline at position 1 in buffer test.org<2>")

Is there a better way to map over first-level heading, 
or alternatively, a better way to set the mark? I tried setting mark to :


 (goto-char (point-min)) (re-search-forward "
\\\*"))

but that gave strange results, with the wrong trees 
being sent to individual files.