diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md
index d6b2e75e1e111127164bd06a7696c35ea8ae40ff..52e154ad1897d4301da5f56567680384d8f5fa10 100644
--- a/guides/source/association_basics.md
+++ b/guides/source/association_basics.md
@@ -460,7 +460,7 @@ class CreatePictures < ActiveRecord::Migration
       t.timestamps null: false
     end
 
-    add_index :pictures, :imageable_id
+    add_index :pictures, [:imageable_id, :imageable_type]
   end
 end
 ```