[highlight_field] Expected START_OBJECT bu (truncated

elasticsearch 使用 highlight 报错

public function searchAction()
    {
        $searchText = $this->getRequest()->getParam('query');
        $result = array('success' => false, 'message' => '');
        $client = new Client();

        $url = 'http://127.0.0.1:9200/job_name/_search';
        $searchArr = [
            "from" => 0,
            "size" => 5,
            "query" => [
                "bool" => [
                    "must" => [
                        [
                            "multi_match" => [
                                "query"  => $searchText
                            ]
                        ]
                    ],
                    "filter" => [
                        ["term" =>  ["external.tenantId" => 49]]
                    ]
                ]
            ]
            ,
            "highlight" => [
                "pre_tags" => ["<b class=\"c_color\">"],
                "post_tags" => ["</b>"],
                "fields" => [
                    "content" => (object)[],
                    "external.bookTitle" => (object)[],
                    "external.bookUploadedBy" => (object)[],
                    "external.bookAssignedGroups" => (object)[]
                ]
            ]
        ];
        $r = $client->request('POST ', $url, [
            "json" => $searchArr
        ]);
        echo $r->getStatusCode(); // 200
        echo $r->getReasonPhrase();
        $result =  $r->getBody()->getContents();

        // print_r('<br><pre>');
        print_r($result);
        // print_r('<hr>');
        // print_r('</pre><br>');
        die;
        // return true;
        $this->_helper->json->sendJson($result);
        // die;
    }

报错是因为

"fields" => [
                    "content" => (object)[],
                    "external.bookTitle" => (object)[],
                    "external.bookUploadedBy" => (object)[],
                    "external.bookAssignedGroups" => (object)[]
                ]