diff --git a/app/helpers/courses_helper.rb b/app/helpers/courses_helper.rb
index da7a34a98..27a973213 100644
--- a/app/helpers/courses_helper.rb
+++ b/app/helpers/courses_helper.rb
@@ -711,21 +711,23 @@ module CoursesHelper
   #课程动态的描述
   def course_activity_desc activity
     desc = ""
-    case activity.course_act_type
-      when "Course"
-        desc = ""
-      when "HomeworkCommon"
-        desc = activity.course_act.description
-      when "News"
-        desc = activity.course_act.description
-      when "Attachment"
-        desc = ""
-      when "Message"
-        desc = activity.course_act.content
-      when "JournalsForMessage"
-        desc = ""
-      when "Poll"
-        desc = activity.course_act.polls_description
+    if activity.course_act
+      case activity.course_act_type
+        when "Course"
+          desc = ""
+        when "HomeworkCommon"
+          desc = activity.course_act.description
+        when "News"
+          desc = activity.course_act.description
+        when "Attachment"
+          desc = ""
+        when "Message"
+          desc = activity.course_act.content
+        when "JournalsForMessage"
+          desc = ""
+        when "Poll"
+          desc = activity.course_act.polls_description
+      end
     end
     desc.html_safe
   end