diff --git a/application/controllers/Caldav.php b/application/controllers/Caldav.php
index 6363450c41..7b1aae4570 100644
--- a/application/controllers/Caldav.php
+++ b/application/controllers/Caldav.php
@@ -150,7 +150,7 @@ public static function sync(string $provider_id): void
         // Sync each appointment with CalDAV Calendar by following the project's sync protocol (see documentation).
 
         foreach ($local_events as $local_event) {
-            if (str_contains($local_event['id_caldav_calendar'], 'RECURRENCE')) {
+            if (str_contains((string) $local_event['id_caldav_calendar'], 'RECURRENCE')) {
                 continue;
             }