diff --git a/app/controllers/report_controller.rb b/app/controllers/report_controller.rb
--- a/app/controllers/report_controller.rb
+++ b/app/controllers/report_controller.rb
@@ -196,6 +196,7 @@
     @struggle = Array.new
     record = {}
     Submission.includes(:problem,:user).order(:problem_id,:user_id).find_each do |sub|
+      next unless sub.user and sub.problem
       if user != sub.user_id or problem != sub.problem_id
         @struggle << { user: record[:user], problem: record[:problem], tries: tries } unless solve
         record = {user: sub.user, problem: sub.problem}
diff --git a/app/views/graders/list.html.haml b/app/views/graders/list.html.haml
--- a/app/views/graders/list.html.haml
+++ b/app/views/graders/list.html.haml
@@ -66,7 +66,7 @@
           %td= sub.try(:user).try(:full_name)
           %td= sub.try(:problem).try(:full_name)
           %td= "#{time_ago_in_words(sub.submitted_at)} ago"
-          %td= "#{time_ago_in_words(sub.graded_at)} ago"
+          %td= "#{sub.graded_at ? time_ago_in_words(sub.graded_at) : ''} ago"
           %td= sub.grader_comment