Routing Error: No route matches {:action=>"show", :controller=>"link_devices", :id=>nil}

I am getting this error, it seems like Link_device 'id can be null. But i checked the database, i did not see any link_device id is nil;

Any suggestions?

<th>
    <%= dns_record_set.name %>
    <small>
        <% if dns_record_set.interface %>
            <%= link_to dns_record_set.interface.link_device, link_device_path(dns_record_set.interface.link_device) %>
        <% end %>
    </small>
</th>