diff --git a/erpnext/accounts/doctype/sales_invoice/sales_invoice.py b/erpnext/accounts/doctype/sales_invoice/sales_invoice.py
index 4de8773..bb74a02 100644
--- a/erpnext/accounts/doctype/sales_invoice/sales_invoice.py
+++ b/erpnext/accounts/doctype/sales_invoice/sales_invoice.py
@@ -1755,15 +1755,10 @@
 		item.purchase_order = parent_child_map.get(sales_item_map.get(item.delivery_note_item))
 
 def get_delivery_note_details(internal_reference):
-	so_item_map = {}
-
 	si_item_details = frappe.get_all('Delivery Note Item', fields=['name', 'so_detail'],
 		filters={'parent': internal_reference})
 
-	for d in si_item_details:
-		so_item_map.setdefault(d.name, d.so_detail)
-
-	return so_item_map
+	return {d.name: d.so_detail for d in si_item_details if d.so_detail}
 
 def get_sales_invoice_details(internal_reference):
 	dn_item_map = {}