The problem is in the pagination of a class based on generic.list.ListView in Django 1.10

During the pagination process, it displays the following error:

"TypeError at / goods / 5 / object of type 'NoneType' has no len ()"

views.py

class GoodsListView( PageNumberView, ListView, SortMixin, CategoryListMixin ): model = Good template_name = 'goods_index.html' paginate_by = 10 cat = None def get( self, request, *args, **kwargs ): if self.kwargs[ 'pk' ] == None: self.cat = Category.objects.first() else: self.cat = Category.objects.get( pk = self.kwargs[ 'pk' ] ) return super( GoodsListView, self ).get( request, *args, **kwargs ) def get_context_data( self, **kwargs ): context = super( GoodsListView, self ).get_context_data( **kwargs ) context[ 'category' ] = self.cat return context 

models.py

 class Good( models.Model ): name = models.CharField( max_length = 50, unique = True, db_index = True, verbose_name = 'Название' ) category = models.ForeignKey( Category, verbose_name = 'Категория' ) description = models.TextField( verbose_name = 'Краткое описание' ) content = models.TextField( verbose_name = 'Полное описание' ) price = models.FloatField( db_index = True, verbose_name = 'Цена, руб.' ) price_acc = models.FloatField( null = True, blank = True, verbose_name = 'Цена с учетом скидки, руб.' ) in_stock = models.BooleanField( default = True, db_index = True, verbose_name = 'Есть в наличии' ) featured = models.BooleanField( default = False, db_index = True, verbose_name = 'Рекомендуемый' ) image = models.ImageField( upload_to = 'goods/list', verbose_name = 'Основное изображение' ) 

urls.py

 urlpatterns = [ url( r'^(?P<pk>\d+)/$', GoodsListView.as_view(), name = 'goods_index' ), ]