newbie question: issue on creating blob field on mysql 5.x for windows

I tried to do a migration task as follows:
lass CreateAttachmentBlob < ActiveRecord::Migration
  def self.up

add_column :posts, :attachment_thumbnail , :binary, :null=>true, :limit=>0.5.megabyte
  end

  def self.down
    remove_column :posts, :attachment_thumbnail
  end
end

when I do rake migrate, the following error occurred:
Mysql::Error: #42000You have an error in your SQL syntax; check the
manual that
corresponds to your MySQL server version for the right syntax to use
near '52428
8.0)' at line 1: ALTER TABLE posts ADD `attachment_thumbnail`
blob(524288.0)

Anyone can tell me what happened? I am using rails 1.2.6 now on ruby
1.8.6 and mysql 5.0.45 GPL.