diff --git a/lib/tasks/public_message.rake b/lib/tasks/public_message.rake
index 6c867241b..2636a454c 100644
--- a/lib/tasks/public_message.rake
+++ b/lib/tasks/public_message.rake
@@ -54,11 +54,11 @@ namespace :sync do
     end
 
     if subject_id.to_i == -1
-      discusses = Shixun.where("parent_id is nul and dis_id=?", shixun_id)
+      discusses = Shixun.where("parent_id is null and dis_id=?", shixun_id)
     else
       shixun_ids = Shixun.find_by_sql("select shixun_id from stage_shixuns where stage_id in (select id from stages where
                                      subject_id=#{subject_id}) ").map(&:shixun_id)
-      discusses = Discuss.where("parent_id is nul and dis_id=?", shixun_ids)
+      discusses = Discuss.where("parent_id is null and dis_id=?", shixun_ids)
     end
 
     discusses.each do |discuss|