diff --git a/db/migrate/20190823023738_change_exericse_1936_scores.rb b/db/migrate/20190823023738_change_exericse_1936_scores.rb
index 202909276..d1fab00f3 100644
--- a/db/migrate/20190823023738_change_exericse_1936_scores.rb
+++ b/db/migrate/20190823023738_change_exericse_1936_scores.rb
@@ -3,7 +3,7 @@ class ChangeExericse1936Scores < ActiveRecord::Migration[5.2]
     #1936的试卷成绩有问题。
     # #https://www.educoder.net/courses/2935/exercises/1936/users/pizfnr5ts
     ex_special = Exercise.find_by_id(1936)
-    ex_special_users = ex_special.exercise_users.exercise_user_committed.where("end_at is not null and end_at > ?",Time.now - 2.months)
+    ex_special_users = ex_special&.exercise_users&.exercise_user_committed&.where("end_at is not null and end_at > ?",Time.now - 2.months)
     if ex_special.present? && ex_special_users.exists?
       ex_special_users.each do |ex_user|
         calculate_score = calculate_student_score(ex_special,ex_user.user)[:total_score]