heroku postgres migration error (which was not occuring on development with sqlite)

Question

I'm trying run a set of migrations on heroku. This is the beginning of the error.

rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::UndefinedTable: ERROR:  relation "articles" does not exist
: ALTER TABLE "settings" ADD CONSTRAINT "fk_rails_679ac32a85"
FOREIGN KEY ("article_id")
  REFERENCES "articles" ("id")

There are two migrations.

The first creates a settings table which references an Articles that has not been created (the next migration is supposed to make this table). The first migration looks like this:

class CreateSettings < ActiveRecord::Migration
  def change
    create_table :settings do |t|
      t.string :commentaryid
      t.string :logo
      t.text :title
      t.text :bannermessage
      t.boolean :blog
      t.string :default_ms_image
      t.string :dark_color
      t.string :light_color
      t.string :commentarydirname
      t.references :article, index: true
      t.timestamps null: false
    end
    add_foreign_key :settings, :articles
  end
end

Then the articles table is supposed to be created in the next migration file:

class CreateArticles < ActiveRecord::Migration
  def change
    create_table :articles do |t|
      t.string :article_name
      t.string :xml_file
      t.string :xslt_file
      t.references :setting, index: true
      t.timestamps null: false
    end
    add_foreign_key :articles, :settings
  end
end

What I don't understand is that I didn't have any trouble with this when I ran this migration on my development machine using sqlite.

So what's the deal.