diff --git a/common.php b/common.php
index c8938d75..32c98e38 100644
--- a/common.php
+++ b/common.php
@@ -17,10 +17,10 @@
  * environment and ensures other functions are loaded.
  */
 
-if(!defined('POSTFIXADMIN')) {
+if(!defined('POSTFIXADMIN')) { # already defined if called from setup.php
     session_start();
+    define('POSTFIXADMIN', 1); # checked in included files
 }
-define('POSTFIXADMIN', 1); # checked in included files
 
 $incpath = dirname(__FILE__);
 (ini_get('magic_quotes_gpc') ? ini_set('magic_quotes_runtime', '0') : '1');
diff --git a/setup.php b/setup.php
index a62d8e2d..94144b55 100644
--- a/setup.php
+++ b/setup.php
@@ -23,6 +23,8 @@
  * Form POST \ GET Variables: -none-
  */
 
+define('POSTFIXADMIN', 1); # by defining it here, common.php will not start a session.
+
 require_once('common.php');
 
 $CONF['show_header_text'] = 'NO';