Nested property fields do not show up using the Reform gem

I'm using the Reform gem to make a form object in my current project but the nested fields don't show up in the form. Here's my code:

Shipment Model:

class Shipment < ApplicationRecord
  has_one :shipment_detail
end

ShipmentDetail Model:

class ShipmentDetail < ApplicationRecord
  belongs_to :shipment
end

Reform Class

class ShipmentForm < Reform::Form
  property :shipment_type
  property :measure

  property :shipment_detail do
    property :po_number
    property :job_no
  end
end

Controller

class ShipmentsController < ApplicationController
  def new
    @shipment = ShipmentForm.new(Shipment.new)
  end
end

Template

<%= form_for @shipment, url: shipments_path, method: :post do |f| %>
  <%= f.label :shipment_type %><br />
  <%= f.text_field :shipment_type %><br /><br />

  <%= f.label :measure %><br />
  <%= f.text_field :measure %><br /><br />

  <%= f.fields_for :shipment_detail do |d| %>
    <%= d.label :po_number %><br />
    <%= d.text_field :po_number %><br /><br />

    <%= d.label :job_no %>
    <%= d.text_field :job_no %><br /><br />
  <% end %>
<% end %>

Only fields shipment_type and measure are visible on the form, po_number and job_no are not. What should I do to make them visible?