I couldn't find an error can anyone help me with this. I dont understand what the error here.

**Invalid schema configuration: User is not a valid type at path ref.

MatchList Schema

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const {ObjectId} = mongoose.Schema;

const match = new Schema({
    type: ObjectId,
    ref:'User'
},{
    timestamp:true
});
const matchList = new Schema({
    user:{
        type: ObjectId,
        ref:'User'
    },
    matches:[match]    
    },{
        timestamp:true
    });

const MatchList = mongoose.model('Match',matchList);

module.exports = MatchList;

UserSchema

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var UserSchema = new Schema({
    firstname:{
        type:String,
        required:true
    },
    lastname:{
        type:String,
        required:true
    },
    email:{
        type:String,
        required:true,
        unique:true
    },
    imgUrl:{
        type:String
    },
    age:{
        type:Number,
        required:true,
        min:16
    },
    gender:{
        type:String,
        required:true
    },
    place:{
        type:String,
    }
},{
    timestamp:true
});

const User = mongoose.model('User',UserSchema);
module.exports = User;