diff --git a/erpnext/controllers/buying_controller.py b/erpnext/controllers/buying_controller.py
index b686dc0..3f2d339 100644
--- a/erpnext/controllers/buying_controller.py
+++ b/erpnext/controllers/buying_controller.py
@@ -838,9 +838,10 @@
 		if not self.get("items"):
 			return
 
-		earliest_schedule_date = min([d.schedule_date for d in self.get("items")])
-		if earliest_schedule_date:
-			self.schedule_date = earliest_schedule_date
+		if any(d.schedule_date for d in self.get("items")):
+			# Select earliest schedule_date.
+			self.schedule_date = min(d.schedule_date for d in self.get("items")
+							if d.schedule_date is not None)
 
 		if self.schedule_date:
 			for d in self.get('items'):