diff --git a/proto/invoice/invoice.proto b/proto/invoice/invoice.proto
index e56253d..90fc874 100644
--- a/proto/invoice/invoice.proto
+++ b/proto/invoice/invoice.proto
@@ -29,6 +29,7 @@
     int64 days_due = 8;
     string iban = 9;
     string swift = 10;
+    string unit = 13;
 }
 
 message CreateInvoiceRequest {