Index: webform.module
===================================================================
--- webform.module	(revision 40781)
+++ webform.module	(working copy)
@@ -1554,7 +1554,8 @@
  * Implements hook_mail().
  */
 function webform_mail($key, &$message, $params) {
-  $message['headers'] = array_merge($message['headers'], $params['headers']);
+  if (is_array ($params['headers']))
+    $message['headers'] = array_merge($message['headers'], $params['headers']);
   $message['subject'] = $params['subject'];
   $message['body'][] = $params['message'];
 }