Mongoose ValidationError upon creating new user in Passport KeystoneJS

Question

I'm using the Passport authentication plugin for KeystoneJS in my project.

Everything works great if the user account exists and is tied to the social network being used. When creating a new user, though, with the config option 'auto create user': true, I end up with a 500 error on the oauth callback route. The log says it's a validation error.

ValidationError: Validation failed 
  at model.Document.invalidate (/app/node_modules/keystone/node_modules/mongoose/lib/document.js:1021:32) 
  at /app/node_modules/keystone/node_modules/mongoose/lib/document.js:970:16 
  at validate (/app/node_modules/keystone/node_modules/mongoose/lib/schematype.js:610:7) 
  at /app/node_modules/keystone/node_modules/mongoose/lib/schematype.js:627:9 
  at Array.forEach (native) 
  at SchemaString.SchemaType.doValidate (/app/node_modules/keystone/node_modules/mongoose/lib/schematype.js:614:19) 
  at /app/node_modules/keystone/node_modules/mongoose/lib/document.js:968:9 
  at process._tickCallback (node.js:419:13)

What could be causing this?

---

Edit

User model:

var keystone = require('keystone'),
Types = keystone.Field.Types,
social = require('keystone-social-login');

/**
 * User Model
 * ==========
 */

var User = new keystone.List('User');

User.add({
    name: { type: Types.Name, required: true, index: true },
    email: { type: Types.Email, initial: true, required: true, index: true },
    password: { type: Types.Password, initial: true, required: true }
}, 'Permissions', {
    userLevel: { type: Types.Select, options: 'user, business, admin', default: 'user', initial: true }
});

// Provide access to Keystone
User.schema.virtual('canAccessKeystone').get(function() {
    return this.userLevel === 'admin';
});


/**
 * Registration
 */

social.plugin(User);

User.defaultColumns = 'name, email, userLevel';
User.register();