title: Joi: "tel" is not allowed to be empty
body: <p>Joi is returning the following error even though <code>tel</code> is set to be <strong>optional</strong>. How do we fix this?</p>
  
  <p>Thanks.</p>
  
  <blockquote>
    <p>Error: Joi Failed: ValidationError: child "tel" fails because ["tel" is not allowed to be empty]</p>
  </blockquote>
  
  <hr>
  
  <pre><code>//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)   
  </code></pre>