TypeError: item.set is not a function

Expected behavior

Setting a name field via User.updateItem should update the field and return no errors.

Actual/Current behavior

Field is not updated, and error is returned:

TypeError: item.set is not a function
    at name.updateItem (/Users/davidm/development/js/tic-manager/node_modules/keystone/fields/types/name/NameType.js:208:9)
    at doFieldUpdate (/Users/davidm/development/js/tic-manager/node_modules/keystone/lib/list/updateItem.js:149:20)
    at /Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:3025:16
    at eachOfArrayLike (/Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:941:9)
    at eachOf (/Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:991:5)
    at Object.eachLimit (/Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:3089:3)
    at /Users/davidm/development/js/tic-manager/node_modules/keystone/lib/list/updateItem.js:176:10
    at /Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:3686:9
    at replenish (/Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:882:17)
    at iterateeCallback (/Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:867:17)
    at /Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:840:16
    at /Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:3691:13
    at apply (/Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:21:25)
    at /Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:56:12
    at /Users/davidm/development/js/tic-manager/node_modules/keystone/lib/list/updateItem.js:171:5
    at /Users/davidm/development/js/tic-manager/node_modules/async/dist/async.js:356:16

Steps to reproduce the actual/current behavior

This is on a super simple 'update profile' style form. I'm learning by screwing around at the moment, so may be user error rather than an actual bug. :)

profile.js:

var keystone = require('keystone');
var User = keystone.list('User');

exports = module.exports = function (req, res) {

    var view = new keystone.View(req, res);
    var locals = res.locals;
    
    // Set locals
	locals.section = 'profile';
	locals.formData = req.body || {};
	locals.validationErrors = {};
    locals.profileUpdated = false;

    view.query('currentUser', User.model.findById(req.user._id));

	view.on('post', { action: 'updateProfile' }, function (next) {
        var userObj = User.model.findById(req.user._id);

        User.updateItem(userObj, req.body, {
            fields: 'name, email',
            user: userObj,
        }, function (err) {
            if (err) {
                console.log(err);
                console.log(req.body);
            } else {
                locals.profileUpdated = true;
            }
            next();
        });
    });
    
    view.render('profile');
  };

profile.pug:

extends ../layouts/default

block intro
	.container
		h1 #{user.username} - Profile
		
block content
	.container
		if profileUpdated
			h3 Your profile has been updated.
		else
			.row: .col-sm-8.col-md-6
				form(method='post')
					input(type='hidden', name='action', value='updateProfile')
					.form-group(class=validationErrors.name ? 'has-error' : null)
						label First Name
						input(type='text', name='name.first', value=user.name.first||'').form-control
					.form-group(class=validationErrors.name ? 'has-error' : null)
						label Last Name
						input(type='text', name='name.last', value=user.name.last||'').form-control
					.form-group(class=validationErrors.email ? 'has-error' : null)
						label Email
						input(type='email', name='email', value=user.email).form-control
					.form-actions
						button(type='submit').btn.btn-primary Send

Environment

Software	Version
Keystone	4.0.0-beta.5
Node	v8.4.0
OS	macOS Sierra
mongoDB	v3.6.1