validates_presence_of

I'm confused about the usage of 'validates_presence_of' and the
'default' parameter of the 'column' method. Let's say I have a model
that looks something like this:

  class CreateErrorTypes < ActiveRecord::Migration
    def self.up
      create_table :error_types do |t|
        t.column :description, :string, :limit => 254, :null => false
        t.column :can_delete, :boolean, :null => false, :default =>
false
      end
    end
    def self.down
      drop_table :error_types
    end
  end

  class ErrorType < ActiveRecord::Base
    validates_presence_of :description
    validates_presence_of :can_delete
  end

Then in console I do this:

  >> ErrorType.create(:description => "General error")

What I expect is an ErrorType object with can_delete set to false, but
what I get is an error saying that can_delete "can't be blank". Why
isn't it setting the default value of that field to false??