diff --git a/routes/billing.js b/routes/billing.js
index 97f7fc6..781c7d8 100644
--- a/routes/billing.js
+++ b/routes/billing.js
@@ -57,7 +57,7 @@ router.get('/', requireAdmin, async (req, res) => {
       FROM invoices i
       JOIN memberships m ON i.membership_id = m.id
       LEFT JOIN tariffs t ON m.tariff_id = t.id
-      WHERE i.period = ?
+      WHERE CONVERT(i.period USING utf8mb4) = CONVERT(? USING utf8mb4)
       ORDER BY m.last_name ASC
     `, [period]);
 
@@ -69,7 +69,7 @@ router.get('/', requireAdmin, async (req, res) => {
         SUM(CASE WHEN status='paid'      THEN 1 ELSE 0 END) as paid_count,
         SUM(CASE WHEN status='open'      THEN amount ELSE 0 END) as open_amount,
         SUM(CASE WHEN status='paid'      THEN amount ELSE 0 END) as paid_amount
-      FROM invoices WHERE period = ?
+      FROM invoices WHERE CONVERT(period USING utf8mb4) = CONVERT(? USING utf8mb4)
     `, [period]);
 
     // Vorschau: Mitglieder die noch keine Rechnung für diese Periode haben
@@ -81,7 +81,7 @@ router.get('/', requireAdmin, async (req, res) => {
         AND m.contract_start <= LAST_DAY(STR_TO_DATE(CONCAT(?, '-01'), '%Y-%m-%d'))
         AND m.contract_end   >= STR_TO_DATE(CONCAT(?, '-01'), '%Y-%m-%d')
         AND m.id NOT IN (
-          SELECT membership_id FROM invoices WHERE period = ?
+          SELECT membership_id FROM invoices WHERE CONVERT(period USING utf8mb4) = CONVERT(? USING utf8mb4)
         )
     `, [period, period, period]);
 
@@ -111,7 +111,7 @@ router.post('/run', requireAdmin, async (req, res) => {
   try {
     // Bereits existierende Rechnungen für diesen Monat prüfen
     const [existing] = await db.query(
-      'SELECT COUNT(*) as c FROM invoices WHERE period = ?', [period]
+      'SELECT COUNT(*) as c FROM invoices WHERE CONVERT(period USING utf8mb4) = CONVERT(? USING utf8mb4)', [period]
     );
     if (existing[0].c > 0) {
       return res.redirect(`/admin/billing?period=${period}&error=Abrechnungslauf+für+${period}+wurde+bereits+durchgeführt`);
@@ -193,7 +193,7 @@ router.post('/mark-all-paid', requireAdmin, async (req, res) => {
   const period = req.body.period || currentPeriod();
   try {
     const [result] = 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 CONVERT(period USING utf8mb4) = CONVERT(? USING utf8mb4) AND status='open'",
       [period]
     );
     res.redirect(`/admin/billing?period=${period}&success=${result.affectedRows}+Rechnungen+als+bezahlt+markiert`);
@@ -212,7 +212,7 @@ router.get('/export/csv', requireAdmin, async (req, res) => {
       SELECT i.*, m.first_name, m.last_name, m.email
       FROM invoices i
       JOIN memberships m ON i.membership_id = m.id
-      WHERE i.period = ? AND i.status = 'open' AND i.amount > 0
+      WHERE CONVERT(i.period USING utf8mb4) = CONVERT(? USING utf8mb4) AND i.status = 'open' AND i.amount > 0
       ORDER BY m.last_name ASC
     `, [period]);