diff --git a/usr/local/www/system_advanced.php b/usr/local/www/system_advanced.php
index 9d487614bd..871eedb193 100755
--- a/usr/local/www/system_advanced.php
+++ b/usr/local/www/system_advanced.php
@@ -167,8 +167,12 @@ if ($_POST) {
 			$config['diag']['ipv6nat']['enable'] = true;
 			$config['diag']['ipv6nat']['ipaddr'] = $_POST['ipv6nat_ipaddr'];
 		} else {
-			unset($config['diag']['ipv6nat']['enable']);
-			unset($config['diag']['ipv6nat']['ipaddr']);
+			if($config['diag']) {
+				if($config['diag']['ipv6nat']) {
+					unset($config['diag']['ipv6nat']['enable']);
+					unset($config['diag']['ipv6nat']['ipaddr']);				
+				}
+			}
 		}
 		$oldcert = $config['system']['webgui']['certificate'];
 		$oldkey = $config['system']['webgui']['private-key'];