Index: /mods/wpsg_mod_prepayment.class.php
===================================================================
--- /mods/wpsg_mod_prepayment.class.php	(revision 8338)
+++ /mods/wpsg_mod_prepayment.class.php	(revision 8339)
@@ -239,4 +239,6 @@
 				$oCountryB = wpsg_country::getInstance($this->shop->get_option('wpsg_shopdata_country'));
 				$oCountryC = wpsg_country::getInstance($kundendaten['land']); 
+				$besrId = '';
+				if (!empty($this->shop->get_option('wpsg_shopdata_bank_besr'))) $besrId = $this->shop->get_option('wpsg_shopdata_bank_besr'); // should be empty if bank = "Postfinance", see also tooltip
 
 					$qrBill = QrBill\QrBill::create();
@@ -271,5 +273,5 @@
 					);
 					$referenceNumber = QrBill\Reference\QrPaymentReferenceGenerator::generate(
-						$this->shop->get_option('wpsg_shopdata_bank_besr')??'null', 
+						$besrId, 
 						$kundendaten['onr']
 					);