diff --git a/app/views/challenges/_challenges_left_nav.html.erb b/app/views/challenges/_challenges_left_nav.html.erb
index 6d2389bb..3111266d 100644
--- a/app/views/challenges/_challenges_left_nav.html.erb
+++ b/app/views/challenges/_challenges_left_nav.html.erb
@@ -1,6 +1,6 @@
 <div class="bor-bottom-greyE clearfix pl20 pr20 pt40 pb20 edu-back-white challengeNav">
   <a href="<%= shixun_challenges_url(@shixun) %>" class="<%= params[:action] == "index" ? "active" : "" %> fl mr40">任务</a>
-  <% if @shixun.propaedeutics.present? %>
+  <% if @shixun.try(:propaedeutics).present? %>
     <a href="<%= propaedeutics_shixun_path(@shixun) %>" class="<%= params[:action] == "propaedeutics" ? "active" : "" %> fl mr40">背景知识</a>
   <% end %>
   <% if User.current.manager_of_shixun?(@shixun) %>