使用DocuSign创建信封时提示收件人用户名无效的问题求助

大家好，我最近在用DocuSign API对接业务时遇到了一个奇怪的问题：明明已经传递了收件人的用户名，但调用接口时却收到错误提示：

The user name for the recipient is invalid. Envelope definition is missing a UserName for recipientId

我把相关代码贴出来，麻烦各位帮忙排查一下问题所在：

第一个函数：获取签名人姓名并调用发送方法 #

public function validate(Request $request)
{
    $valideur = User::find($devi->id_assigne);

    $signer2 = array(
        "firstname" =>   $valideur->firstname,
        "lastname" => $valideur->lastname,
    );

    $name = $signer2["firstname"];
    $this->sends($name);
}

第二个函数：创建DocuSign信封及收件人视图 #

public function sends($name)
{
    $this->args = $this->getTemplateArgs();
    $args = $this->args;
    $envelope_args = $args["envelope_args"];

    $envelope_definition = $this->makeEnvelopeFileObject($args["envelope_args"],$name);

    $apiClient = new ApiClient();
    $apiClient->getOAuth()->setOAuthBasePath(config('app.auth_server'));

    try{
        $accessToken = $this->getToken($apiClient);
    } catch (\Throwable $th) {
        return back()->withError($th->getMessage())->withInput();
    }

    $userInfo = $apiClient->getUserInfo($accessToken);
    $accountInfo = $userInfo[0]->getAccounts();
    $apiClient->getConfig()->setHost($accountInfo[0]->getBaseUri() . config('app.sufix'));
    $apiClient->getConfig()->addDefaultHeader('Authorization', 'Bearer ' . $args['ds_access_token']);

    $envelopeApi = new EnvelopesApi($apiClient);
    $results = $envelopeApi->createEnvelope($accountInfo[0]->getAccountId(), $envelope_definition);
    $envelope_id = $results->getEnvelopeId();

    $authentication_method = 'None'; # How is this application authenticating
    $recipient_view_request = new \DocuSign\eSign\Model\RecipientViewRequest([
        'authentication_method' => $authentication_method,
        'client_user_id' => '1000',
        'recipient_id' => '1',
        'return_url' => $envelope_args['ds_return_url'],
        'user_name' => $name  , 
        'email' =>''
    ]);

    $results = $envelopeApi->createRecipientView($accountInfo[0]->getAccountId(), $envelope_id,
        $recipient_view_request);
}

我已经dump过$name变量，能得到正确的名（firstname），但就是搞不懂为什么会触发这个错误。希望有经验的朋友能帮我看看哪里处理错了！

备注：内容来源于stack exchange，提问作者Rubyjane