invalid_response_exception for Optional single structure

Hi,

I have a webservice which should return a single record with different values or an empty value

public static function find_me_returns()

{
    $entry = new external_single_structure(
        array(
            'id' => new external_value(PARAM_INT, 'Id of the user'),
            'idnumber' => new external_value(PARAM_TEXT, 'External id number of the user'),
            'suspended' => new external_value(PARAM_INT, 'Suspended state of the user'),
        ), 'find me values', VALUE_OPTIONAL);

    return entry;
}

The function itself declares an empty return value:

$return_value = array();

So if nothing is found there should be an empty array.

But instead i get: