Django管理员错误，无法导入用户

Question

我有一个models.py，如下所示：

from django.db import models
from django.contrib.auth.models import User

class Change(models.Model):
RFC = models.CharField(max_length=10)
Ticket_Number = models.CharField(max_length=10)
Plan_Owner = models.ForeignKey(User, related_name='plan_owner', unique=True)
Change_Owner = models.ForeignKey(User, related_name='change_owner', unique=True)

我的admin.py如下：

from django.contrib import admin

class ChangeAdmin(admin.ModelAdmin):
search_fields = ('RFC', 'Ticket_Number','Plan_Owner')
list_display = ('RFC', 'Ticket_Number','Plan_Owner')

fieldsets = [
    ('Ticket Details', {
        'fields': ['RFC', 'Ticket_Number']}),
    ('Implementation details', {
        'fields': ['Plan_Owner', 'Change_Owner']})
            ]

admin.site.register(Change, ChangeAdmin)

这给我一个错误说

  

异常值：'tuple'对象不可调用

此处完整追溯：http://pastebin.com/35nsKzQH