make sure default `apply_discount_on` is set when creating invoice
diff --git a/erpnext/accounts/doctype/subscriptions/subscriptions.py b/erpnext/accounts/doctype/subscriptions/subscriptions.py
index ac0310a..bbf2a69 100644
--- a/erpnext/accounts/doctype/subscriptions/subscriptions.py
+++ b/erpnext/accounts/doctype/subscriptions/subscriptions.py
@@ -269,6 +269,8 @@
 		# Discounts
 		if self.apply_additional_discount:
 			invoice.apply_discount_on = self.apply_additional_discount
+		else:
+			invoice.apply_discount_on = 'Grand Total'
 
 		if self.additional_discount_percentage:
 			invoice.additional_discount_percentage = self.additional_discount_percentage