python - 「unicode」オブジェクトには属性「date」がありません

Question

私はこの種のコードを持っています:

models.py :

from django.db import models
import ioweyou.settings as settings
from comment.models import Comment


class Entry(models.Model):

    OPEN = 0
    ACCEPTED = 1
    REJECTED = 2
    DELETED = 3
    STATUS_CHOICES = (
        (OPEN, 'Open'),
        (ACCEPTED, 'Accepted'),
        (REJECTED, 'Rejected'),
        (DELETED, 'Deleted'),
    )

    name = models.CharField(max_length=255)
    description = models.TextField('entry description', blank=True, null=True)
    value = models.DecimalField(max_digits=6, decimal_places=2)
    lender = models.ForeignKey(settings.AUTH_USER_MODEL, null=True, related_name='lender')
    debtor = models.ForeignKey(settings.AUTH_USER_MODEL, null=True, related_name='debtor')
    status = models.SmallIntegerField(default=0, choices=STATUS_CHOICES)
    accepted_at = models.DateTimeField(blank=True, null=True)
    rejected_at = models.DateTimeField(blank=True, null=True)
    created_at = models.DateTimeField(auto_now_add=True, editable=False)
    updated_at = models.DateTimeField(auto_now=True)

    def __unicode__(self):
        return self.name

class EntryComment(Comment):
    entry = models.ForeignKey(Entry, null=False, related_name="comments")

admin.py

class EntryAdmin(admin.ModelAdmin):
    fieldsets = [
        (None,               {'fields': ['name']}),
        ('Date information', {'fields': ['accepted_at']}),
    ]

admin.site.register(models.Entry, EntryAdmin)

そしてこの種のエラー：

AttributeError at /admin/entry/entry/116/
'unicode' object has no attribute 'date'

Error during template rendering
In template /Library/Python/2.7/site-packages/django/contrib/admin/templates/admin/includes/fieldset.html, error at line 19

django と python は最近扱っています。そして、私の知識は完全ではありません。誰でも私を助けることができますか？

Entry を編集しようとして、フィールド Accepted_at の値が決定されている (設定済み) 場合にエラーが発生します。