diff --git a/app/services/zip_service.rb b/app/services/zip_service.rb
index 8864999d..ef71063e 100644
--- a/app/services/zip_service.rb
+++ b/app/services/zip_service.rb
@@ -52,7 +52,7 @@ module ZipService
     members = exercise.course.members
     exercise_users.each_with_index do |exercise_user, index|
       member = members.where(:user_id => exercise_user.user_id).first
-      group_name = member.try(:course_group_id).to_i == 0 ? '未分班' : member.course_group.name
+      group_name = member.try(:course_group_id).to_i == 0 ? '未分班' : member.course_group.try(:name)
       # export_file_name = "#{group_name}-#{exercise.course_id}-#{exercise.exercise_name}-#{exercise_user.user.user_extensions.student_id}-#{exercise_user.user.show_real_name}" + ".pdf"
       export_file_name = "#{exercise.course_id}-#{index+1}" + ".pdf"
       out_file = export_user_exercise(exercise, exercise_user, export_file_name)