diff --git a/erpnext/manufacturing/doctype/workstation/workstation.py b/erpnext/manufacturing/doctype/workstation/workstation.py
index b939803..5095784 100644
--- a/erpnext/manufacturing/doctype/workstation/workstation.py
+++ b/erpnext/manufacturing/doctype/workstation/workstation.py
@@ -60,9 +60,10 @@
 	workstation = frappe.get_doc("Workstation", workstation)
 
 	for working_hour in workstation.working_hours:
-		slot_length = (to_timedelta(working_hour.end_time or "") - to_timedelta(working_hour.start_time or "")).total_seconds()
-		if slot_length >= operation_length:
-			return
+		if working_hour.start_time and working_hour.end_time:
+			slot_length = (to_timedelta(working_hour.end_time or "") - to_timedelta(working_hour.start_time or "")).total_seconds()
+			if slot_length >= operation_length:
+				return
 
 	frappe.throw(_("Operation {0} longer than any available working hours in workstation {1}, break down the operation into multiple operations").format(operation, workstation.name), NotInWorkingHoursError)