diff --git a/application/HistoricBill/HistoricBillModel.php b/application/HistoricBill/HistoricBillModel.php
index 648c5b293..179bc8de5 100644
--- a/application/HistoricBill/HistoricBillModel.php
+++ b/application/HistoricBill/HistoricBillModel.php
@@ -15,7 +15,7 @@ class HistoricBillModel extends TTCrudBaseModel {
     public string $BIC;
     public string $IBAN;
     public int $brutto_sum;
-    public string $fibu_number;
+    public ?string $fibu_number;
     public string $Mandatinvoice_number;
     public int $payment;
-}
\ No newline at end of file
+}