Joi is returning the following error even though tel is set to be optional. How do we fix this?

Thanks.

Error: Joi Failed: ValidationError: child tel fails because [tel is not allowed to be empty]

//Define Joi schema

const schema = {

    email: Joi.string().required().email({

        errorLevel: 64,

        minDomainAtoms: 2 

    }).min(6),

    tel: Joi.string().optional().min(10).max(10),

    password: Joi.string().required().min(8).max(64)

}



//Check inputs

const { error, value } = Joi.validate({ 

    email: args.email, 

    tel: tel, 

    password: args.password 

}, schema)