NoMethodError in Admin/orders#show

Showing admin/orders/show.html.erb where line #9 raised:

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.first_name

Extracted source (around line #9):

6:    <h3>Billing Address</h3>
7: 	<div class="box">
8:     <p>
9:         <strong><% @order.billing_address.first_name %> <% @order.billing_address.last_name %></strong>
10:         <br/>
11:         <%= @order.billing_address.street_1 %>
12:         <br/>