diff --git a/app/models/submission.rb b/app/models/submission.rb
index b8667e68..ea72fe17 100644
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -38,7 +38,7 @@ class Submission < ActiveRecord::Base
   end
 
   def normalized_score
-    if score && exercise.maximum_score && exercise.maximum_score > 0
+    if !score.nil? && !exercise.maximum_score.nil? && (exercise.maximum_score > 0)
       score / exercise.maximum_score
     else
       0