diff --git a/calendar/setup/tables_update.inc.php b/calendar/setup/tables_update.inc.php
index 1c2ea14068..5f3065b8d6 100644
--- a/calendar/setup/tables_update.inc.php
+++ b/calendar/setup/tables_update.inc.php
@@ -2759,6 +2759,7 @@ function calendar_upgrade17_1()
 {
 	// Update birthdays as events preference from boolean
 	$change = function($attr, $old_value, $owner) {
+		if (!isset($old_value)) return null;	// do not set anything, if nothing was set before
 		return $old_value ? 'birthday' : 'none';
 	};
 	Api\Preferences::change_preference('calendar', 'birthdays_as_events', $change);