diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index f1fbae2a..93b731fa 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -94,10 +94,6 @@ module ActiveModel
 
         define_include_method attr
 
-        if self.method_defined? :_fast_attributes
-          undef :_fast_attributes
-        end
-
       end
 
       def associate(klass, attrs) #:nodoc: