Django /Related字段处的FieldError获得无效查找：为null

我正在创建注释api，但运行服务器时出现以下错误：

字段错误在/ 相关字段的查找无效：是否为空

我不知道怎么修理它。我正在创建一个嵌套的注释api。这是我的代码：

#序列化程序

class CommentSerializer(serializers.ModelSerializer):
    loadParent = serializers.SerializerMethodField("loadPrentData")

    def loadPrentData(self, comment):
        comments = Comment.objects.filter(parent=comment)
        comments_ser = CommentSerializer(comments, many=True).data
        return comments_ser

    class Meta:
        model = Comment
        fields = ['id', 'user', 'product', 'parent', 'body', 'created', 'loadParent']


class ProductSerializer(serializers.ModelSerializer):
    comments = serializers.SerializerMethodField("loadProductComments")

    def loadProductComments(self, _product):
        _comments = Comment.objects.filter(product=_product, parent__is_null=True)
        _comments_ser = CommentSerializer(_comments, many=True, read_only=True).data
        return _comments_ser

    class Meta:
        model = Product
        fields = ['id', 'category', 'name', 'slug', 'image_1',
                  'image_2', 'image_3', 'image_4', 'image_5',
                  'description', 'price', 'available', 'created', 'updated', 'comments']
        lookup_field = 'slug'
        extra_kwargs = {
            'url': {'lookup_field': 'slug'}
        }

#观点：

你的线路

_comments = Comment.objects.filter(product=_product, parent__is_null=True)

应该是

_comments = Comment.objects.filter(product=_product, parent__isnull=True)

_comments = Comment.objects.filter(product=_product, parent__isnull=True)