diff --git a/src/Models/Invoice.php b/src/Models/Invoice.php
index 6396e0b..20741e0 100644
--- a/src/Models/Invoice.php
+++ b/src/Models/Invoice.php
@@ -237,12 +237,15 @@ class Invoice
         $invoice = self::find($id);
         if (!$invoice) return;
 
-        if (!empty($invoice['beleg_id'])) {
-            Database::execute('DELETE FROM bookings WHERE beleg_id = ?', [$invoice['beleg_id']]);
-            Database::execute('DELETE FROM belege WHERE id = ?', [$invoice['beleg_id']]);
-        }
+        $belegId = !empty($invoice['beleg_id']) ? (int)$invoice['beleg_id'] : null;
+
         Database::execute('DELETE FROM invoice_lines WHERE invoice_id = ?', [$id]);
         Database::execute('DELETE FROM invoices WHERE id = ?', [$id]);
+
+        if ($belegId !== null) {
+            Database::execute('DELETE FROM bookings WHERE beleg_id = ?', [$belegId]);
+            Database::execute('DELETE FROM belege WHERE id = ?', [$belegId]);
+        }
     }
 
     public static function generateRechnungsNr(): string