diff --git a/lib/shoulda/general.rb b/lib/shoulda/general.rb
index ef44ac6d..66e9f3f6 100644
--- a/lib/shoulda/general.rb
+++ b/lib/shoulda/general.rb
@@ -182,9 +182,20 @@ module ThoughtBot # :nodoc:
       # <tt>value</tt> by making sure the <tt>error_message_to_avoid</tt> is not
       # contained within the list of errors for that attribute.
       #
-      #   assert_good_value(User.new, :email, "user@example.com") #=> passes
-      #   assert_good_value(User.new, :ssn, "123456789", /length/) #=> passes
-      def assert_good_value(object, attribute, value, error_message_to_avoid = //)
+      #   assert_good_value(User.new, :email, "user@example.com")
+      #   assert_good_value(User.new, :ssn, "123456789", /length/)
+      #
+      # If a class is passed as the first argument, a new object will be
+      # instantiated before the assertion.  If an instance variable exists with
+      # the same name as the class (underscored), that object will be used
+      # instead.
+      #
+      #   assert_good_value(User, :email, "user@example.com")
+      #
+      #   @product = Product.new(:tangible => false)
+      #   assert_good_value(Product, :price, "0")
+      def assert_good_value(object_or_klass, attribute, value, error_message_to_avoid = //)
+        object = get_instance_of(object_or_klass)
         object.send("#{attribute}=", value)
         object.valid?
         assert_does_not_contain(object.errors.on(attribute), error_message_to_avoid, "when set to #{value.inspect}")
@@ -194,9 +205,20 @@ module ThoughtBot # :nodoc:
       # <tt>value</tt> by making sure the <tt>error_message_to_expect</tt> is
       # contained within the list of errors for that attribute.
       #
-      #   assert_bad_value(User.new, :email, "invalid") #=> passes
-      #   assert_bad_value(User.new, :ssn, "123", /length/) #=> passes
-      def assert_bad_value(object, attribute, value, error_message_to_expect = /invalid/)
+      #   assert_bad_value(User.new, :email, "invalid")
+      #   assert_bad_value(User.new, :ssn, "123", /length/)
+      #
+      # If a class is passed as the first argument, a new object will be
+      # instantiated before the assertion.  If an instance variable exists with
+      # the same name as the class (underscored), that object will be used
+      # instead.
+      #
+      #   assert_bad_value(User, :email, "invalid")
+      #
+      #   @product = Product.new(:tangible => true)
+      #   assert_bad_value(Product, :price, "0")
+      def assert_bad_value(object_or_klass, attribute, value, error_message_to_expect = /invalid/)
+        object = get_instance_of(object_or_klass)
         object.send("#{attribute}=", value)
         assert !object.valid?, "#{object.class} allowed #{value.inspect} as a value for #{attribute}"
         assert object.errors.on(attribute), "There are no errors on #{attribute} after being set to #{value.inspect}"
@@ -206,6 +228,17 @@ module ThoughtBot # :nodoc:
       def pretty_error_messages(obj)
         obj.errors.map { |a, m| "#{a} #{m} (#{obj.send(a).inspect})" }
       end
+
+      private
+
+      def get_instance_of(object_or_klass)
+        if object_or_klass.is_a?(Class)
+          klass = object_or_klass
+          instance_variable_get("@#{klass.to_s.underscore}") || klass.new
+        else
+          object_or_klass
+        end
+      end
     end
   end
 end
diff --git a/test/fixtures/products.yml b/test/fixtures/products.yml
new file mode 100644
index 00000000..e69de29b
diff --git a/test/other/helpers_test.rb b/test/other/helpers_test.rb
index 0478233f..e3dd098e 100644
--- a/test/other/helpers_test.rb
+++ b/test/other/helpers_test.rb
@@ -129,6 +129,20 @@ class HelpersTest < Test::Unit::TestCase # :nodoc:
         assert_good_value User.new, :ssn, "x", /length/
       end
     end
+
+    should "accept a class as the first argument" do
+      assert_good_value User, :email, "good@example.com"
+    end
+
+    context "with an instance variable" do
+      setup do
+        @product = Product.new(:tangible => true)
+      end
+
+      should "use that instance variable" do
+        assert_good_value Product, :price, "9999", /included/
+      end
+    end
   end
 
   context "assert_bad_value" do
@@ -151,5 +165,19 @@ class HelpersTest < Test::Unit::TestCase # :nodoc:
         assert_bad_value User.new, :ssn, "xxxxxxxxx", /length/
       end
     end
+
+    should "accept a class as the first argument" do
+      assert_bad_value User, :email, "bad"
+    end
+
+    context "with an instance variable" do
+      setup do
+        @product = Product.new(:tangible => true)
+      end
+
+      should "use that instance variable" do
+        assert_bad_value Product, :price, "0", /included/
+      end
+    end
   end
 end
diff --git a/test/rails_root/app/models/product.rb b/test/rails_root/app/models/product.rb
new file mode 100644
index 00000000..1108a44d
--- /dev/null
+++ b/test/rails_root/app/models/product.rb
@@ -0,0 +1,9 @@
+class Product < ActiveRecord::Base
+  validates_presence_of :title, :price
+  validates_inclusion_of :price, :in => 0..99, :unless => :tangible
+
+  validates_inclusion_of :price, :in => 1..9999, :if => :tangible
+  validates_inclusion_of :weight, :in => 1..100, :if => :tangible
+  validates_format_of :size, :with => /.+x.+x.+/, :if => :tangible
+  validates_length_of :size, :in => 5..20, :if => :tangible
+end
diff --git a/test/rails_root/db/migrate/009_create_products.rb b/test/rails_root/db/migrate/009_create_products.rb
new file mode 100644
index 00000000..173f948f
--- /dev/null
+++ b/test/rails_root/db/migrate/009_create_products.rb
@@ -0,0 +1,17 @@
+class CreateProducts < ActiveRecord::Migration
+  def self.up
+    create_table :products do |t|
+      t.string :title
+      t.integer :price
+      t.integer :weight
+      t.string :size
+      t.boolean :tangible
+
+      t.timestamps
+    end
+  end
+
+  def self.down
+    drop_table :products
+  end
+end