angular – Reactive Forms双向绑定错误：无法分配给只读属性

export interface User {
  name: string;
  subscribed: boolean;
}

export class UserEditComponent implements OnInit {
  modifiedUser: User;
  userForm: FormGroup;

  ngOnInit() {
    this.userForm = this.fb.group({
      name: ['',Validators.required],subscribed: false
    });

    this.route.paramMap.switchMap((params: ParamMap) => {
      return this.userService.getUser(params.get('id'));
    }).subscribe((user) => {
      this.modifiedUser = user;
      this.userForm.setValue({
        name: this.modifiedUser.name,subscribed: this.modifiedUser.subscribed
      });
    });

    this.userForm.valueChanges.subscribe((data) => {
      this.modifiedUser.subscribed = data.subscribed;
    });
  }
}

<form [formGroup]="userForm">
  <textarea class="form-control" formControlName="name">{{modifiedUser.name}}</textarea>
  <input type="checkbox" class="custom-control-input" formControlName="subscribed">
</form>

但是,我总是得到错误TypeError：一旦表单出现,就不能在控制台中分配给对象'[object Object]’的只读属性’subscribed’.知道为什么吗？