Rails: Convert has_many Association to has_one Association

I have 2 models: TripPlan and Place. The models are like below:

# app/models/trip_plan.rb
class TripPlan < ActiveRecord::Base
  has_many :places
end

# app/models/place.rb
class Place < ActiveRecord::Base
  belongs_to :trip_plan
end

There's a corresponding migration for the places table as well:

class CreatePlaces < ActiveRecord::Migration
  def change
    create_table :places do |t|
      t.references :trip_plan, index: true

      t.timestamps null: false
    end
  end
end

So each TripPlan can have several places and each Place belongs to a single trip plan. But now I need a has_one/belongs_to relation between these models. I have modified TripPlan model as follows:

class TripPlan < ActiveRecord::Base
  has_one :place
end

But now if I try TripPlan.find(1).place.build it gives an error that undefined method 'build' for nil:NilClass. How can I fix this?

Thanks in advance!