TemplateSyntaxError at /admin/books/book/

Caught AttributeError while rendering: 'str' object has no attribute 'year'

Request Method: 	GET
Request URL: 	http://127.0.0.1:8000/admin/books/book/
Django Version: 	1.2.5
Exception Type: 	TemplateSyntaxError
Exception Value: 	

Caught AttributeError while rendering: 'str' object has no attribute 'year'

Exception Location: 	C:\Python27\lib\site-packages\django-1.2.5-py2.7.egg\django\contrib\admin\templatetags\admin_list.py in date_hierarchy, line 282
Python Executable: 	C:\Python27\python.exe
Python Version: 	2.7.1
Python Path: 	['D:\\WORK\\DJANGO\\mysite', 'C:\\Python27\\lib\\site-packages\\genshi-0.6-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\xlrd-0.7.1-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\xlwt-0.7.2-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\xlutils-1.4.1-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\errorhandler-1.1.1-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\pip-0.8.2-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\pyside-1.0.0qt472-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\rdflib-3.0.0-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\pycim-14.15.2-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\cim-14.12a1-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\pylon-0.4.4-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\simplejson-2.1.3-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\sparqlwrapper-1.4.2-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\ipython-0.10.1-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\pysnmp-4.1.15a-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\pycrypto-2.3-py2.7-win32.egg', 'C:\\Python27\\lib\\site-packages\\pyasn1-0.0.13a-py2.7.egg', 'C:\\Python27\\lib\\site-packages\\django-1.2.5-py2.7.egg', 'C:\\Program Files\\JetBrains\\PyCharm 1.1.1\\helpers', 'D:\\WORK\\DJANGO\\mysite', 'D:\\WORK\\DJANGO', 'C:\\WINDOWS\\system32\\python27.zip', 'C:\\Python27\\DLLs', 'C:\\Python27\\lib', 'C:\\Python27\\lib\\plat-win', 'C:\\Python27\\lib\\lib-tk', 'C:\\Python27', 'C:\\Python27\\lib\\site-packages']
Server time: 	Чтв, 17 Мар 2011 14:17:10 +0300
Template error

In template c:\python27\lib\site-packages\django-1.2.5-py2.7.egg\django\contrib\admin\templates\admin\change_list.html, error at line 77
Caught AttributeError while rendering: 'str' object has no attribute 'year'
67 	{% endif %}
68 	{% endblock %}
69 	{% if cl.formset.errors %}
70 	<p class="errornote">
71 	{% blocktrans count cl.formset.errors|length as counter %}Please correct the error below.{% plural %}Please correct the errors below.{% endblocktrans %}
72 	</p>
73 	{{ cl.formset.non_form_errors }}
74 	{% endif %}
75 	<div class="module{% if cl.has_filters %} filtered{% endif %}" id="changelist">
76 	{% block search %}{% search_form cl %}{% endblock %}
77 	{% block date_hierarchy %}{% date_hierarchy cl %}{% endblock %}
78 	
79 	{% block filters %}
80 	{% if cl.has_filters %}
81 	<div id="changelist-filter">
82 	<h2>{% trans 'Filter' %}</h2>
83 	{% for spec in cl.filter_specs %}{% admin_list_filter cl spec %}{% endfor %}
84 	</div>
85 	{% endif %}
86 	{% endblock %}

from django.db import models

class Publisher(models.Model):
	name = models.CharField(max_length=30)
	address = models.CharField(max_length=50)
	city = models.CharField(max_length=60)
	state_province = models.CharField(max_length=30, blank=True)
	country = models.CharField(max_length=50)
	website = models.URLField(blank=True)
	
	def __unicode__(self):
		return self.name
	
class Author(models.Model):
	first_name = models.CharField(max_length=30)
	last_name = models.CharField(max_length=40)
	email = models.EmailField(blank=True, verbose_name='e-mail')
	
	def __unicode__(self):
		return u'%s %s' % (self.first_name, self.last_name)
	
class Book(models.Model):
	title = models.CharField(max_length=100)
	authors = models.ManyToManyField(Author)
	publisher = models.ForeignKey(Publisher)
	publication_date = models.DateField()
	
	def __unicode__(self):
		return self.title

from django.contrib import admin
from mysite.books.models import Publisher, Author, Book

class AuthorAdmin(admin.ModelAdmin):
    list_display = ('first_name', 'last_name', 'email')
    search_fields = ('first_name', 'last_name')

class BookAdmin(admin.ModelAdmin):
    list_display = ('title', 'publisher', 'publication_date')
    list_filter = ('publication_date',)
    date_hierarchy = 'publication_date'
    ordering = ('-publication_date',)
    filter_horizontal = ('authors',)

class PublisherAdmin(admin.ModelAdmin):
    list_display = ('name', 'city', 'website')
    search_fields = ('name',)

admin.site.register(Publisher, PublisherAdmin)
admin.site.register(Author, AuthorAdmin)
admin.site.register(Book, BookAdmin)

date_hierarchy = 'publication_date'