diff --git a/app/api/mobile/entities/activity.rb b/app/api/mobile/entities/activity.rb
index ad4af39fd..fdc03c47b 100644
--- a/app/api/mobile/entities/activity.rb
+++ b/app/api/mobile/entities/activity.rb
@@ -30,22 +30,26 @@ module Mobile
                     ac.nil? || ac.act.nil? ? 0 : ac.act.journals.where("notes is not null and notes != ''").count
                   end
                 when :subject
-                  if ac.act_type == "HomeworkCommon"
-                    ac.act.name unless ac.nil? || ac.act.nil?
-                  elsif ac.act_type == "News" || ac.act_type == "BlogComment"
-                    ac.act.title unless ac.nil? || ac.act.nil?
-                  elsif ac.act_type == "Message" || ac.act_type == "Issue"
-                    ac.act.subject unless ac.nil? || ac.act.nil?
-                  elsif ac.act_type == "JournalsForMessage"
-                    ac.act.private == 0 ? "留言" : "私信"
+                  unless ac.act.nil?
+                    if ac.act_type == "HomeworkCommon"
+                      ac.act.name unless ac.nil? || ac.act.nil?
+                    elsif ac.act_type == "News" || ac.act_type == "BlogComment"
+                      ac.act.title unless ac.nil? || ac.act.nil?
+                    elsif ac.act_type == "Message" || ac.act_type == "Issue"
+                      ac.act.subject unless ac.nil? || ac.act.nil?
+                    elsif ac.act_type == "JournalsForMessage"
+                      ac.act.private == 0 ? "留言" : "私信"
+                    end
                   end
                 when :description
-                  if ac.act_type == "HomeworkCommon" || ac.act_type == "Issue" || ac.act_type == "News"
-                    ac.act.description unless ac.nil? || ac.act.nil?
-                  elsif ac.act_type == "Message" || ac.act_type == "BlogComment"
-                    ac.act.content unless ac.nil? || ac.act.nil?
-                  elsif ac.act_type == "JournalsForMessage"
-                    ac.act.notes
+                  unless ac.act.nil?
+                    if ac.act_type == "HomeworkCommon" || ac.act_type == "Issue" || ac.act_type == "News"
+                      ac.act.description unless ac.nil? || ac.act.nil?
+                    elsif ac.act_type == "Message" || ac.act_type == "BlogComment"
+                      ac.act.content unless ac.nil? || ac.act.nil?
+                    elsif ac.act_type == "JournalsForMessage"
+                      ac.act.notes
+                    end
                   end
                 when :latest_update
                   time_from_now ac.updated_at unless ac.nil?
@@ -103,12 +107,14 @@ module Mobile
         if a.is_a? ::UserActivity
           if a.act_type == "ProjectCreateInfo"
             get_user(get_project(a.act_id).user_id)
-          elsif a.act_type == 'Issue' || a.act_type == 'News' || a.act_type == 'Message' || a.act_type == 'BlogComment'
+          elsif !a.act.nil?
+            if a.act_type == 'Issue' || a.act_type == 'News' || a.act_type == 'Message' || a.act_type == 'BlogComment'
             a.act.author
-          elsif a.act_type == 'HomeworkCommon' || a.act_type == 'Poll' || a.act_type == 'JournalsForMessage'
-            a.act.user
-          elsif a.act_type == 'Course'
-            a.act.teacher
+            elsif a.act_type == 'HomeworkCommon' || a.act_type == 'Poll' || a.act_type == 'JournalsForMessage'
+              a.act.user
+            elsif a.act_type == 'Course'
+              a.act.teacher
+            end
           end
         end
       end