diff --git a/lib/active_model_serializers/adapter/attributes.rb b/lib/active_model_serializers/adapter/attributes.rb
index c062127c..8281392b 100644
--- a/lib/active_model_serializers/adapter/attributes.rb
+++ b/lib/active_model_serializers/adapter/attributes.rb
@@ -55,7 +55,7 @@ module ActiveModelSerializers
       def serializable_hash_for_single_resource(options)
         resource = resource_object_for(options)
         relationships = resource_relationships(options)
-        resource.merge!(relationships)
+        resource.merge(relationships)
       end
 
       def resource_relationships(options)
diff --git a/test/support/rails_app.rb b/test/support/rails_app.rb
index 1dc7e506..67b7e607 100644
--- a/test/support/rails_app.rb
+++ b/test/support/rails_app.rb
@@ -8,7 +8,7 @@ module ActiveModelSerializers
       # TODO: figure out why turning on the memory cache changes
       # the result of the CacheTest#test_associations_cache_when_updated
       # and if it is more correct or less correct.
-      # config.action_controller.cache_store = :memory
+      config.action_controller.cache_store = :memory_store
     end
 
     app.routes.default_url_options = { host: 'example.com' }