Index: latex.el
===================================================================
RCS file: /sources/auctex/auctex/latex.el,v
retrieving revision 5.481
diff -u -u -r5.481 latex.el
--- latex.el	17 Jan 2013 07:20:42 -0000	5.481
+++ latex.el	27 Jan 2013 10:29:23 -0000
@@ -1100,8 +1100,8 @@
 (defun LaTeX-env-contents (environment)
   "Insert ENVIRONMENT with filename for contents."
   (save-excursion
-    (when (re-search-backward "^\\\\documentclass.*{" nil t)
-      (error "Put %s environment before \\documentclass" environment)))
+    (when (re-search-backward LaTeX-header-end nil t)
+      (error "Put %s environment before \\begin{document}" environment)))
   (LaTeX-insert-environment environment
 			    (concat TeX-grop
 				    (read-string "File: ")