为什么 django-mptt 显示错误：不支持的操作数类型 -：'str' 和 'str'？

Question

models.py ->

class Category(MPTTModel):
    title = models.CharField(max_length=200)
    description = models.TextField(blank=True, null=True)
    parent = TreeForeignKey(
        'self', blank=True, null=True, related_name='children', on_delete=models.CASCADE)

    class MPTTMeta:
        order_insertion_by = ['title']

class Card(models.Model):
    categories = TreeManyToManyField(Category, related_name='category_cards', blank=True, default=None)

admin.py ->

@admin.register(Category)
class CategoryAdmin2(DraggableMPTTAdmin):
    list_display = ('tree_actions', 'indented_title',)
    list_display_links = ('indented_title',)

在管理界面中，我收到此错误：

TypeError at /admin/myapp/category/3/change/
unsupported operand type(s) for -: 'str' and 'str'

如何解决？