diff --git a/activemodel/test/cases/serializeration/json_serialization_test.rb b/activemodel/test/cases/serializeration/json_serialization_test.rb
index 91e123e6554dccd81e9c2e2bf367213c7985bd0e..3bc39bb06d949022f5aed2b96c72f54b53f4b1c9 100644
--- a/activemodel/test/cases/serializeration/json_serialization_test.rb
+++ b/activemodel/test/cases/serializeration/json_serialization_test.rb
@@ -1,5 +1,6 @@
 require 'cases/helper'
 require 'models/contact'
+require 'models/automobile'
 require 'active_support/core_ext/object/instance_variables'
 
 class Contact