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Trying to stub the findOne method of my User mongoose model but getting the error Cannot stub non-existent own property findOne .

I am using the findOne method in my connector to fetch the user and that works fine. It is only in the test it says the method doesn't exist:

I can call find one like so:

const user = await UserModel
        .findOne({email: email})
        .select(['hash', 'firstName', 'lastName', 'email'])
and it returns my user with the selected attributes.
When I try override it in my test: 
import UserModel from '../../models/user'
beforeEach(() => {
  // ...
  myStub = sinon
    .stub(UserModel.prototype, 'findOne')
    .callsFake(() => Promise.resolve(expectedUser))
Is there any reason why sinon would think my UserModel.prototype doesn't have the method findOne()?
My user model
import { isEmail } from 'validator';
import mongoose, { ObjectId } from 'mongoose';
import bcrypt from 'bcrypt';
const Schema = mongoose.Schema;
const UserSchema = new Schema({
    firstName: { type : String, required : true },
    lastName: { type : String, required : true },
    email: { type: String, trim: true, lowercase: true, unique: true, required: 'Email address is required', validate: [ isEmail, 'invalid email' ] },
    hash: { type : String, required : true, select: false },
    refreshToken: { type : String, select: false },
// generate hash
UserSchema.methods.generateHash = (password) => bcrypt.hashSync(password, bcrypt.genSaltSync(8), null)
// checking if password is valid
UserSchema.methods.validPassword = (password, hash) => bcrypt.compareSync(password, hash)
export default mongoose.model('User', UserSchema);
                Is UserModel a variable? It looks like a class. In that case you would have to write new UserModel. But if it's a variable and not an instance of anything, findOne is not a property of UserModel.prototype.
– Uncreative Name
                Aug 11, 2018 at 17:36
                @UncreativeName I have updated my post to show my userModel. It is an instance of a mongoose model
– Stretch0
                Aug 11, 2018 at 17:39
                In other test I am running, I can succesfully stub the .save() method of the UserModel so I am not sure what's different about the .findOne() method. Perhaps because I am also calling .select()?
– Stretch0
                Aug 11, 2018 at 17:43
UserModel doesn't have findOne property neither as it's own property, nor in the prototype as soon as it inherits it from Model (mongoose.Model). 
You can access original findOne method via UserModel.__proto__.findOne.
        
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