diff --git a/routes/billing.js b/routes/billing.js
index 998440f..3f94fff 100644
--- a/routes/billing.js
+++ b/routes/billing.js
@@ -217,8 +217,9 @@ router.get('/export/csv', requireAdmin, async (req, res) => {
     `, [period]);
 
     // Alle offenen Rechnungen dieser Periode als bezahlt markieren
+    // COLLATE explizit setzen wegen möglichem Collation-Konflikt
     await db.query(
-      "UPDATE invoices SET status='paid', paid_at=NOW() WHERE period=? AND status='open'",
+      "UPDATE invoices SET status='paid', paid_at=NOW() WHERE period=? COLLATE utf8mb4_unicode_ci AND status='open'",
       [period]
     );