NoMethodError in Main#sell_horse

Showing /home/webapp/findhorsesforsale.com/app/views/main/_part3.html.erb where line #17 raised:

undefined method `paypal_basic_url' for nil:NilClass

Extracted source (around line #17):

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<br />
<br />
<%= link_to image_tag("https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif"), @ad.paypal_basic_url(root_url) %>
</div>
<div class="col-lg-4">
<h3>Premium ad for $20</h3>

Trace of template inclusion: app/views/main/sell_horse.html.erb

Rails.root: /home/webapp/findhorsesforsale.com

Application Trace | Framework Trace | Full Trace
app/views/main/_part3.html.erb:17:in `_app_views_main__part__html_erb__1193648083938772810_47364484241900'
app/views/main/sell_horse.html.erb:50:in `_app_views_main_sell_horse_html_erb___3400133165156970428_47364433580360'

Request

Parameters:

None

Response

Headers:

None