protected function _validate($state)
    {
        // Unique values can't be null or empty string.
        if (empty($state->value) && !is_numeric($state->value)) {
            return false;
        }

        if (is_array($state->value)) {
            // The first element of the array can't be null or empty string.
            $first = array_slice($state->value, 0, 1);
            if (empty($first) && !is_numeric($first)) {
                return false;
            }
        }

        return true;
    }