使用URL2中的url传递数据会导致错误

Question

this.params={
  "id": this.id,
  "fields": "language,largeIcon,shortDescription,subtitleLanguagesCsv,video,videoId,aboutTheCourse,faq,courseSyllabus,courseFormat,suggestedReadings,instructor,estimatedClassWorkload,recommendedBackground,sessions.fields(durationString),instructors.fields(photo150,fullName,title,department,middleName),universities.fields(name,locationCity,logo,squareLogo),categories.fields(id,name)",
  "includes": "universities,categories,instructors,sessions"
};

 this.courseraService.view("courses",this.params)
     .subscribe (response => {
         this.courseDetails = response.elements[0];
      }, error =>  this.errorMessage = <any>error);



view(model,data) {

    this.url = this.courseraUrl + model;

    let params: URLSearchParams = new URLSearchParams();
        for (let key in data) {
                  params.set(key, data[key]);
        }
    let options = new RequestOptions({search: params});

    return this.http.get(this.url ,options)
        .map ( response => response.json())
        // .do( data => console.log ('view: ' + JSON.stringify(data) ) )
                  .catch(this.handleError);
  }

这显示了尝试使用url传递的数据中的错误。错误是：

Cannot read property 'photo150' of undefined