diff --git a/fusion_shipping/api/ups_rest/request.py b/fusion_shipping/api/ups_rest/request.py
index 2c0990a1..e7a57617 100644
--- a/fusion_shipping/api/ups_rest/request.py
+++ b/fusion_shipping/api/ups_rest/request.py
@@ -431,9 +431,11 @@ class UPSRequest:
             (ship_from.country_id.code == 'US' and ship_to.country_id.code == 'US') or
             (ship_from.country_id.code == 'PR' and ship_to.country_id.code == 'PR')
         ):
-            request['ShipmentRequest']['Shipment']['ReferenceNumber'] = {
-                'Value': shipment_info.get('reference_number')
-            }
+            # reference_number is already a list of {'Code', 'Value'} dicts
+            # (see _ups_rest_prepare_shipping_data). UPS expects ReferenceNumber
+            # to be such an object (or an array of them) with a string Value --
+            # NOT {'Value': [<dict>, ...]}, which UPS rejects on the ship call.
+            request['ShipmentRequest']['Shipment']['ReferenceNumber'] = shipment_info.get('reference_number')
 
         # Shipments from US to CA or PR require extra info
         if ship_from.country_id.code == 'US' and ship_to.country_id.code in ['CA', 'PR']: