I have a table 'invoices' in my development database (sqlite3) populated with a small amount of test data.

I wanted to add a column 'invoice_number' to it and set up a migration like so:

class AddInvoiceNumberColumnToInvoices < ActiveRecord::Migration
  def self.up
    add_column :invoices, :invoice_number, :integer
  end

  def self.down
    remove_column :invoices, :invoice_number
  end
end

I ran rake db:migrate and it seemed to migrate just fine. However, when I tried to access this column through ActiveRecord it didn't seem to be there. I decided to undo this migration and try again (not sure what I was going to try but I thought I'd start by undoing it) with rake db:migrate VERSION='whatever_the_migration_before_this_one_was_called'. This failed with the error message

==  AddInvoiceNumberColumnToInvoices: reverting ===============================
-- remove_column(:invoices, :invoice_number)
rake aborted!
An error has occurred, this and all later migrations canceled:

altered_invoices.invoice_number may not be NULL

I can't find any documentation of this error. Is anyone able to explain what I have done wrong, and more importantly how I can fix this?