Inline dans Django admin: n'a pas d'ForeignKey

J'ai deux modèles Django:

class Author(models.Model):
    user_email =  models.CharField(max_length=100, blank=True)
    display_name =  models.CharField(max_length=250)

class Photo(models.Model):
    author = models.ForeignKey(Author)
    image = ThumbnailImageField(upload_to='photos')

Pour obtenir inline photos, j'ai en admin.py:

class PhotoInline(admin.StackedInline):
    model = Author

class AuthorAdmin(admin.ModelAdmin):
    list_display = ('display_name','user_email')
    inlines = [PhotoInline]

J'obtiens une erreur: Exception at /admin/metainf/author/11/

<class 'metainf.models.Author'> has no ForeignKey to <class 'metainf.models.Author'>

Pourquoi?

OriginalL'auteur user2140513 | 2014-02-20