'Value must be a string in the XXXXXXXX formats.'

def clean(self, value):

    value = super(PEDNIField, self).clean(value)
    if (value in EMPTY_VALUES):
        return u''
    if (not value.isdigit()):
        raise ValidationError(self.error_messages[u'invalid'])
    if (len(value) != 8):
        raise ValidationError(self.error_messages[u'max_digits'])
    return value
