Mysql2::Error: All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead

I have created a demo application in rails 3.2.9 and ruby versiion 2.0.0 . After scaffolding Blog model I am trying to migrate it, but having following issue.

# rake db:migrate

==  CreateBlogs: migrating ====================================================
-- create_table(:blogs)
rake aborted!
StandardError: An error has occurred, all later migrations canceled:

Mysql2::Error: All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead: CREATE TABLE `blogs` (`id` int(11) DEFAULT NULL auto_increment PRIMARY KEY, `title` varchar(255), `description` text, `created_at` datetime NOT NULL, `updated_at` datetime NOT NULL) ENGINE=InnoDB
/usr/local/rvm/gems/ruby-2.0.0-p648@demo-app/gems/activerecord-3.2.9/lib/active_record/connection_adapters/abstract_mysql_adapter.rb:245:in `query'
/usr/local/rvm/gems/ruby-2.0.0-p648@demo-app/gems/activerecord-3.2.9/lib/active_record/connection_adapters/a

bstract_mysql_adapter.rb:245:in `block in execute'
/usr/local/rvm/gems/ruby-2.0.0-p648@demo-app/gems/activerecord-
-3.2.9/lib/active_record/migration.rb:551:in `migrate'
/usr/local/rvm/gems/ruby-2.0.0-p648@demo-app/gems/activerecord-3.2.9/lib/active_record/railties/databases.rake:179:in `block (2 levels) in <top (required)>'
/usr/local/rvm/gems/ruby-2.0.0-p648@demo-app/gems/rake-11.3.0/exe/rake:27:in `<top (required)>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)