谁能帮我我为什么会出现TypeError？

Question

let express = require('express'), 
router  = express.Router(), 
passport = require('passport');  

let Artistrees = require('../models/artistree'); 


//INDEX - get/find all artists 
router.get('/', (req, res) => {
   Artistrees.find({}, (err, allArtists) => {
       if(err) {
           console.log(err); 
       } else {
           res.render('artistrees/index', {allArtists: allArtists});     
       }
   });  
});

结果：

  

TypeError：Artistrees.find不是函数