diff --git a/README.md b/README.md
index ea16f82..1136200 100644
--- a/README.md
+++ b/README.md
@@ -17,9 +17,9 @@ user.tutorials[:quick_start] = :visited     # => NoMethodError: undefined method
 
 There are two ways to solve this problem - a. break down `options` into multiple columns like `tutorials` and `preference`, or b. define an accessor method for each to initialize with an empty `Hash` when accessed for the first time.
 
-The former is bad because the TEXT (or BLOB) column type could be [stored off-page](http://www.mysqlperformanceblog.com/2010/02/09/blob-storage-in-innodb/) when it gets big and you could hit some strange bugs and/or performance penalty. Furthermore, adding columns kills the primary purpose of having key-value store - you use this feature because you don't like migrations, right? So it's two-fold bad.
+The former is bad because the TEXT (or BLOB) column type could be stored off-page when it gets big and you could hit some [strange bugs and/or performance penalty](http://www.mysqlperformanceblog.com/2010/02/09/blob-storage-in-innodb/). Furthermore, adding columns kills the primary purpose of having key-value store - you use this feature because you don't like migrations, right? So it's two-fold bad.
 
-StoreField takes the latter approach. It defines accessors that initializes with an empty `Hash` or `Set` automatically. Now you have a single TEXT column for everything!
+StoreField takes the latter approach. It defines accessors that initialize with an empty `Hash` or `Set` automatically. Now you have a single TEXT column for everything!
 
 ## Usage
 
@@ -78,9 +78,21 @@ cart.funnel                     # => #<Set: {:add_item, :checkout}>
 cart.set_funnel(:checkout).save!    # => true
 ```
 
+Also you can enumerate acceptable values, which is validated on the `set_[field]` method.
+
+```ruby
+store_field :funnel, type: Set, values: [ :add_item, :checkout ]
+```
+
+With the definition above, the following code will raise an exception.
+
+```ruby
+set_funnel(:bogus)  # => ArgumentError: :bogus is not allowed
+```
+
 ## Use cases for the Set type
 
-Set is a great way to store an arbitrary number of states.
+Set is a great way to store an arbitrary number of named states.
 
 Consider you have a system that sends an alert when some criteria have been met.
 
diff --git a/lib/store_field.rb b/lib/store_field.rb
index f1e5889..f9a40eb 100644
--- a/lib/store_field.rb
+++ b/lib/store_field.rb
@@ -7,26 +7,32 @@ module StoreField
 
   module ClassMethods
     def store_field(key, options = {})
-      raise ArgumentError.new(':in is invalid')   if options[:in] and serialized_attributes[options[:in].to_s].nil?
-      raise ArgumentError.new(':type is invalid') if options[:type] and ![ Hash, Set ].include?(options[:type])
+      raise ArgumentError.new(':in is invalid')     if options[:in] and serialized_attributes[options[:in].to_s].nil?
+      raise ArgumentError.new(':type is invalid')   if options[:type] and ![ Hash, Set ].include?(options[:type])
+      raise ArgumentError.new(':values is invalid') if options[:values] and !options[:values].is_a?(Array)
 
       klass = options[:type]
+      values = options[:values]
       store_attribute = options[:in] || serialized_attributes.keys.first
       raise ArgumentError.new('store method must be defined before store_field') if store_attribute.nil?
 
       # Accessor
       define_method(key) do
-        value = send("#{store_attribute}")[key]
+        value = send(store_attribute)[key]
         if value.nil?
           value = klass ? klass.new : {}
-          send("#{store_attribute}")[key] = value
+          send(store_attribute)[key] = value
         end
         value
       end
 
       # Utility methods for Set
       if options[:type] == Set
-        define_method("set_#{key}")     {|value| send(key).add(value); self }
+        define_method("set_#{key}") do |value|
+          raise ArgumentError.new("#{value.inspect} is not allowed") if values and !values.include?(value)
+          send(key).add(value)
+          self
+        end
         define_method("unset_#{key}")   {|value| send(key).delete(value); self }
         define_method("set_#{key}?")    {|value| send(key).include?(value) }
         define_method("unset_#{key}?")  {|value| !send(key).include?(value) }
diff --git a/lib/store_field/version.rb b/lib/store_field/version.rb
index c41aec5..52785c2 100644
--- a/lib/store_field/version.rb
+++ b/lib/store_field/version.rb
@@ -1,3 +1,3 @@
 module StoreField
-  VERSION = '1.0.0'
+  VERSION = '1.1.0'
 end
diff --git a/spec/store_field_spec.rb b/spec/store_field_spec.rb
index 4e28c1b..c30e563 100644
--- a/spec/store_field_spec.rb
+++ b/spec/store_field_spec.rb
@@ -7,7 +7,7 @@
 class User < ActiveRecord::Base
   store :storage
   store_field :tutorials
-  store_field :delivered, type: Set
+  store_field :delivered, type: Set, values: [ :welcome, :balance_low ]
 end
 
 describe StoreField do
@@ -30,19 +30,22 @@ class User < ActiveRecord::Base
     @user.delivered.should == Set.new
   end
 
+  it 'raises when invalid value is given for Set' do
+    expect {
+      @user.set_delivered(:bogus)
+    }.to raise_error(ArgumentError)
+  end
+
   it 'sets and unsets keywords' do
     @user.set_delivered(:welcome)
-    @user.set_delivered(:first_deposit)
 
     # Consume balance, notify once and only once
     @user.set_delivered(:balance_low)
-    @user.set_delivered(:balance_negative)
 
     # Another deposit, restore balance
     @user.unset_delivered(:balance_low)
-    @user.unset_delivered(:balance_negative)
 
-    @user.delivered.should == Set.new([:welcome, :first_deposit])
+    @user.delivered.should == Set.new([:welcome])
   end
 
   it 'saves in-line' do