Угловой 6: невозможно прочитать свойство length of undefined

<quiz-course [quiz] = "currentUnitQuiz"></quiz-course>

export class CoursePlayComponent implements OnInit {
  errorMessage: string;
  course: ICourse;
  courseId: number;

  // the current and prev quiz of this and prev unit (if there are)
  public currentUnitQuiz: IQuiz;
  public showQuiz: boolean;
  
  // .. more properties


  constructor(private courseService: CourseService,
      private route: ActivatedRoute,
      private router: Router,
      private userProgressService: UserProgressService) { }

  ngOnInit() {
      // save this course id from course-detail and get http request from the service
      this.courseId = JSON.parse(localStorage.getItem("courseId"));
      this.getCourse(this.courseId);
    }
    
  // Get course detail by id
  getCourse(id: number) {
      this.courseService.getCourse(id).subscribe(
          course => {
            this.course = course;
            
            // .. more code

            if (this.route.snapshot.firstChild.data.type == 'quiz') {
              this.getQuiz(this.currentUnitPos);
            }
          },
          error  => this.errorMessage = <any>error);
      }
      
      getQuiz(currentUnitPosition: number) {
      // .. more code
        this.showQuiz = true;
        this.currentUnitQuiz = this.course.units.find(u => u.position ===
        currentUnitPosition).quiz;
      }
 }

export class CourseQuizComponent implements OnInit {

  @Input() quiz: IQuiz;
  
  // 1 = show the first page. 2 = show questions page. 0 = show neither
  public currentQuestion: IQuestion;
  public currentIndex: number;
  public currentUnit: number;
  public userAnswers: number[] = [];
  public correctAnswers: number;

  constructor(private courseService: CourseService,
      private route: ActivatedRoute,
      private router: Router,
      private fb: FormBuilder) { }

  ngOnInit() {
    console.info("length in child: ", this.quiz.questions.length); // **NOT** undefined
    this.restartQuiz();
  }
  
    restartQuiz() {
    this.currentUnit = +this.route.snapshot.paramMap.get('unitId');
    this.correctAnswers = 0;

    for(let i = 0; i < this.quiz.questions.length; i++) {
      this.userAnswers[i] = 0;
    }

    this.getCurrentQuestion(0);
  }

}

export interface IQuiz {
  id: number;
  name: number;
  unit_id: number;
  unit_position: number;
  questions: IQuestion[];
}

export interface IQuestion {
  id: number;
  name: string;
  quiz_id: number;
  position: number;
  question: string;
  answer1: string;
  answer2: string;
  answer3: string;
  answer4: string;
  correct: number;
  selected: number;
}