diff --git a/erpnext/selling/doctype/sales_order/sales_order.py b/erpnext/selling/doctype/sales_order/sales_order.py
index 084a218..f1b56d9 100644
--- a/erpnext/selling/doctype/sales_order/sales_order.py
+++ b/erpnext/selling/doctype/sales_order/sales_order.py
@@ -104,8 +104,8 @@
 	def validate_delivery_date(self):
 		if self.order_type == 'Sales':
 			if not self.delivery_date:
-				self.delivery_date = max([d.delivery_date for d in self.get("items") if d.delivery_date])
-
+				delivery_date_list = [d.delivery_date for d in self.get("items") if d.delivery_date]
+				self.delivery_date = max(delivery_date_list) if delivery_date_list else None
 			if self.delivery_date:
 				for d in self.get("items"):
 					if not d.delivery_date: