角度2-TypeError：无法读取属性

Question

我是Angular的新手。运行角度代码时，控制台出现以下错误。

AppComponent.html:2 ERROR TypeError: Cannot read property 'colourPreference' of undefined
    at AppComponent.get [as colour] (app.component.ts:22)
    at Object.eval [as updateDirectives] (AppComponent.html:6)
    at Object.debugUpdateDirectives [as updateDirectives] (services.ts:443)
    at checkAndUpdateView (view.ts:359)

app.componenet.ts：

import { Component, OnInit } from '@angular/core';
import { UserPreferencesService } from './UserPreferences.service';
import { FormsModule } from '@angular/forms';
@Component({
  selector: 'my-app',
  template: `
  <h1>This is the home page</h1>
  <div>
  Colour preferences:
  <input type='text' [(ngModel)]='colour' [style.background]='colour'/>
  </div>
  `
})
export class AppComponent { 
  constuctor(private _userPreferencesService: UserPreferencesService){

  }
  get colour():string{
    return this._userPreferencesService.colourPreference;
  }

  set colour(value: string){
    this._userPreferencesService.colourPreference=value;
  }
 }

UserPreferences.service.ts

import {Injectable} from '@angular/core';

@Injectable()
export class UserPreferencesService{
colourPreference: string ='orange';
}

有人可以解决上述问题吗？谢谢！！