title: Django admin.E110 Error - The value of 'list_display_links' must be a list, a tuple, or None
body: <p>Just upgrade my app to Django 1.9.1 and am now getting an <code>admin.E110</code> error.  I've searched but can't figure out what the correction is.  I was missing trailing commas, but after adding them, it was still broken.  Below is some of the code, if you could please take a look.  Thank you in advance.</p>
  
  <p><strong>admin.py</strong></p>
  
  <pre><code>from django.contrib import admin
  from adminsortable2.admin import SortableAdminMixin
  from .models import Listing
  
  
  class ListingAdmin(SortableAdminMixin, admin.ModelAdmin):
      list_display = ['title', 'listing_name', 'location_type', 'featured_choices', 'status_choices', ]
      search_fields = ['title', 'listing_name', ]
      pass
  
  
  admin.site.register(Listing, ListingAdmin)
  </code></pre>