diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index 1431228..626c8b0 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -289,8 +289,9 @@ module ActiveModel
       end
 
       defaults = @base.class.lookup_ancestors.map do |klass|
-        [ :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.underscore}.attributes.#{attribute}.#{type}",
-          :"#{@base.class.i18n_scope}.errors.models.#{klass.model_name.underscore}.#{type}" ]
+        model_scope = klass.model_name.underscore.gsub '/', '.'
+        [ :"#{@base.class.i18n_scope}.errors.models.#{model_scope}.attributes.#{attribute}.#{type}",
+          :"#{@base.class.i18n_scope}.errors.models.#{model_scope}.#{type}" ]
       end
 
       defaults << options.delete(:message)
diff --git a/activemodel/test/cases/validations/i18n_validation_test.rb b/activemodel/test/cases/validations/i18n_validation_test.rb
index e9f0e43..ce4b415 100644
--- a/activemodel/test/cases/validations/i18n_validation_test.rb
+++ b/activemodel/test/cases/validations/i18n_validation_test.rb
@@ -363,4 +363,17 @@ class I18nValidationTest < ActiveModel::TestCase
     assert_equal ["I am a custom error"], @person.errors[:title]
   end
 
+  # test class i18n scope with Module
+
+  def test_model_with_module_i18n_scope
+      I18n.backend.store_translations 'en', :activemodel => {:errors => {:models => {:person_module => {:person => {:blank => 'generic blank'}}}}}
+      PersonModule::Person.validates_presence_of :title
+      person = PersonModule::Person.new :title => nil
+      person.valid?
+      assert_equal ['generic blank'], person.errors[:title]
+
+      I18n.backend.store_translations 'en', :activemodel => {:errors => {:models => {:person_module => {:person => {:attributes => {:title => {:blank => 'title cannot be blank'}}}}}}}
+      person.valid?
+      assert_equal ['title cannot be blank'], person.errors[:title]
+  end
 end
diff --git a/activemodel/test/models/person.rb b/activemodel/test/models/person.rb
index cf16a38..eb84f7a 100644
--- a/activemodel/test/models/person.rb
+++ b/activemodel/test/models/person.rb
@@ -11,3 +11,8 @@ end
 
 class Child < Person
 end
+
+module PersonModule
+  class Person < ::Person
+  end
+end