Django form_有效错误。有人能找到错误吗?

我做了一个网址 path('nova-serie/<categoria>', NovaSerie.as_view(), name='nova_serie'),

创建新系列的URL如下所示:

我试着用 form_valid 但我收到的信息是:

视图.py

class NovaSerie(CreateView):
    model = Serie
    form_class = SerieForm
    template_name = 'nova_serie.html'
    success_url = reverse_lazy('home')

    def form_valid(self, form):
        url = self.request.path_info
        parte_final_url = url.replace('/nova-serie/', '')
        form.instance.categoria = parte_final_url
        return super(NovaSerie).form_valid(form) 

表单.py

class SerieForm(forms.ModelForm):
    class Meta:
        model = Serie
        fields = (
            'serie',

        )
        widgets = {
            'title': forms.TextInput(),  # attrs={class="title"}
        }

这里有人能帮我个忙吗?