column cannot be null

I'm getting an error leaving a field blank but I have blank = True in
the model

IntegrityError at /admin/afiliados/afiliado/add/
(1048, "Column 'titular_id' cannot be null")
Request Method: POST
Request URL: http://localhost/mysite/admin/afiliados/afiliado/add/
Django Version: 1.3
Exception Type: IntegrityError
Exception Value:
(1048, "Column 'titular_id' cannot be null")

------------------------
class afiliado(models.Model):
....other fields...
titular = models.OneToOneField("self",blank = True)

Thanks!

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