diff --git a/src/psm/Module/Server/Controller/AbstractServerController.php b/src/psm/Module/Server/Controller/AbstractServerController.php
index 0e5d8b4d..53145b66 100644
--- a/src/psm/Module/Server/Controller/AbstractServerController.php
+++ b/src/psm/Module/Server/Controller/AbstractServerController.php
@@ -107,7 +107,8 @@ abstract class AbstractServerController extends AbstractController {
 		$server['last_online'] = psm_timespan($server['last_online']);
 		$server['last_offline'] = psm_timespan($server['last_offline']);
 		if ($server['last_offline'] != psm_get_lang('system', 'never')) {
-			$server['last_offline_duration'] = "(".$server['last_offline_duration'].")";
+			$server['last_offline_duration'] = is_null($server['last_offline_duration']) ?
+                null : "(".$server['last_offline_duration'].")";
 		}
 		$server['last_check'] = psm_timespan($server['last_check']);
 		$server['active'] = psm_get_lang('system', $server['active']);