diff --git a/app/models/conference.rb b/app/models/conference.rb
index f3565814..2b3526b2 100644
--- a/app/models/conference.rb
+++ b/app/models/conference.rb
@@ -591,7 +591,7 @@ class Conference < ActiveRecord::Base
   #
   # Reports an error when such a condition is found
   def valid_date_range?
-    errors.add(:start_date, "Start date is greater than End date") if start_date > end_date
+    errors.add(:start_date, 'Start date is greater than End date') if start_date && end_date && start_date > end_date
   end
 
   ##