diff --git a/README.rdoc b/README.rdoc
index ab3d43a..ece56f1 100644
--- a/README.rdoc
+++ b/README.rdoc
@@ -38,16 +38,17 @@ Then run
   
   rails generate validates_timeliness:install
 
-This creates configuration initializer and locale files. In the initializer, you need to uncomment the extend_classes setting like so
+This creates configuration initializer and locale files. In the initializer, you there are a number of config options to customize the plugin.
 
   ValidatesTimeliness.setup do |config|
 
-    config.extend_classes = [ ActiveRecord::Base ]
+    # Add validation helpers to these classes
+    # config.extend_classes = [ ActiveRecord::Base ]
 
   end
 
-This adds the validation helper methods to ActiveRecord. Replace it with the ORM of your choosing.
-As long as it supports ActiveModel it should work.
+By default the plugin extends ActiveRecord if present. If you are using one or more other ORMs, you need to add them to this config option array.
+As long as the ORM supports ActiveModel it should work.
 
 
 == Usage:
diff --git a/lib/validates_timeliness.rb b/lib/validates_timeliness.rb
index e86ee15..58e2785 100644
--- a/lib/validates_timeliness.rb
+++ b/lib/validates_timeliness.rb
@@ -17,7 +17,7 @@ module ValidatesTimeliness
 
   # Add validation helpers to these classes
   mattr_accessor :extend_classes
-  @@extend_classes = []
+  @@extend_classes = [ defined?(ActiveRecord) && ActiveRecord::Base ].compact
 
   # Set the dummy date part for a time type values.
   mattr_accessor :dummy_date_for_time_type