diff --git a/integration_test_utils.py b/integration_test_utils.py
index 92a15ea..27b765e 100644
--- a/integration_test_utils.py
+++ b/integration_test_utils.py
@@ -810,6 +810,8 @@ def update_checkout_session(
     """
     # Default to existing values if not provided
     currency = currency if currency is not None else checkout_obj.currency
+    # If buyer is not provided, use the existing buyer
+    buyer = buyer if buyer is not None else checkout_obj.buyer
 
     # Construct Line Items
     if line_items is None: