diff --git a/lib/active_model_serializers.rb b/lib/active_model_serializers.rb
index 7066ebf42..3137e6282 100644
--- a/lib/active_model_serializers.rb
+++ b/lib/active_model_serializers.rb
@@ -12,7 +12,16 @@
   ActiveSupport.on_load(:action_controller) do
     if ::ActionController::Serialization.enabled
       ActionController::Base.send(:include, ::ActionController::Serialization)
-      ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+
+      if ActiveSupport::VERSION::STRING < '5'
+        if defined?(Rails) && Rails.env.test?
+          ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+        end
+      else
+        ActiveSupport.on_load(:action_controller_test_case) do
+          ActionController::TestCase.send(:include, ::ActionController::SerializationAssertions)
+        end
+      end
     end
   end
 rescue LoadError