IntegrityError with BooleanField and Postgresql

I have this simple Blog model:

class Blog(models.Model):

title = models.CharField(_('title'), max_length=60, blank=True,
null=True)
body = models.TextField(_('body'))
user = models.ForeignKey(User)
is_public = models.BooleanField(_('is public'), default = True)

When I insert a blog in admin interface, I get this error:

IntegrityError at /admin/blogs/blog/add/

null value in column "is_public" violates not-null constraint

Why ???

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