PHP Soapclient returns object has no property - php

Question

When sending a request from my PHP soapclient as described below, I get:
[getInterestAndExchangeRates failed: Please check inparameters in Xxx].
I suspect that it is the [$response] line that has wrong syntax. More specific the [$searchRequestParameters]. I have checked that all the content in the array is in the correct order.
Question:
Is there something wrong with below code syntax?
I have followed following instructions:
https://swea.riksbank.se/sweaWS/docs/api/call/getInterestAndExchangeRates.htm
<?php
/**
* A soapclient request to Swedish Central bank, API.
*/
$client = new SoapClient("https://swea.riksbank.se/sweaWS/wsdl/sweaWS_ssl.wsdl");
$searchGroupSeries = array(
"groupid" => 2,
"seriesid" => "SECBREPOEFF"
);
$searchRequestParameters = array (
"aggregateMethod" => "W",
"avg" => true,
"datefrom" => "2018-01-01",
"dateto" => "2019-01-01",
"languageid" => "en",
"max" => true,
"min" => false,
"searchGroupSeries" => $searchGroupSeries,
"ultimo" => false
);
// Test 1 (pointing out above array, does not work):
$response = $client->__soapCall("getInterestAndExchangeRates", $searchRequestParameters);
// Test 2 (wrap in above array inside another array, does not work):
// $response = $client->__soapCall('getInterestAndExchangeRates', array('searchRequestParameters' => $searchRequestParameters));
var_dump($response);
Error:
PHP Fatal error: Uncaught SoapFault exception: [soap:Server] getInterestAndExchangeRates failed: Please check inparameters in [path]_(does_not_work).php:22
Stack trace:
#0 [path]_(does_not_work).php(22): SoapClient->__soapCall('getInterestAndE...', Array)
#1 {main}
thrown in [path]_(does_not_work).php on line 22