add index migration adding column instead of migration

I am trying to add a index to a column which already exists in my table by using

I want to add a index on item_id on product_images table

bundle exec rails generate migration AddIndexToProductImages item_id:integer:index

but the code i see in the migration file is

class AddIndexToProductImages < ActiveRecord::Migration
  def change
    add_column :product_images, :item_id, :integer
  end
end

Not sure what could be causing this, can anyone help? Thanks.