Array to string conversion with nusoap client - php

Question

I sent request to webservice like this :
$transaction = new Transaction();
$transaction->amount = $amount;
$transaction->order_id = $orderId;
$transaction->status = 0;
$transaction->additional_data = $additionalData;
$transaction->type = 1;
$transaction->ip = Request::getClientIp();
$transaction->save();
$soap = new nusoap_client(self::$request_url);
$fields = array(
'terminalId' => self::$terminalId,
'userName' => self::$username,
'userPassword' => self::$password,
'orderId' => $transaction->order_id,
'amount' => $amount,
'localDate' => date('Ymd'),
'localTime' => date('His'),
'additionalData' => $additionalData,
'callBackUrl' => self::$callBackUrl,
'payerId' => 0,
);
$response = $soap->call('bpPayRequest', $fields, 'http://interfaces.core.sw.bps.com/');
but i get exception in line 2125 in nusoap.php :
Array to string conversion
exception thrown from :
// TODO: there is a PHP bug that can cause this to SEGV for CURLINFO_CONTENT_TYPE
foreach(curl_getinfo($this->ch) as $k => $v){
$err .= "$k: $v<br>";
}
$this->debug($err);