diff --git a/lib.inc.php3 b/lib.inc.php3
index 5bbfa7739..a1f704599 100755
--- a/lib.inc.php3
+++ b/lib.inc.php3
@@ -247,7 +247,7 @@ if (!defined('__LIB_INC__')){
             // Else ensure the username is not the same
             else {
                 // force user to enter a different username
-                if ($old_usr == $PHP_AUTH_USER) {
+                if (isset($PHP_AUTH_USER) && $old_usr == $PHP_AUTH_USER) {
                     $do_auth = TRUE;
                 } else {
                     $do_auth = FALSE;