Defind a Joi validation with valid() and optional(), default()

I’m using Joi to validate an environment variable that can be either ‘true’ or ‘false’ as strings. The variable is optional, and if it is undefined, I want to set it to ‘true’ by default. Here is the Joi rule I wrote:

Joi.string().valid('true', 'false').default('true')

but validation fails when ENV is missing with this error:

must be one of [true, false]

How can I modify the Joi schema to accept the environment variable as either ‘true’ or ‘false’, and default to ‘true’ if it is not provided?