Joi - error TS2339: Property 'required' does not exist on type 'typeof 'joi''

[RichardJECooke]

This code fails:

export const PostUsersRolesDataValidator : Joi.ObjectSchema = Joi.object().keys({
    roles: Joi.object().required(),
  franchiseeId: Joi.string().min(36).max(36).when('franchisorId', { is: false, then: Joi.required() }),
  franchisorId: Joi.string().min(36).max(36).when('franchiseeId', { is: false, then: Joi.required() })
});

with the error ../shared/joiValidators/PostUsersRolesDataValidator.ts(5,90): error TS2339: Property 'required' does not exist on type 'typeof 'joi''.

Idea from here: hapijs/joi#194