Context

• node version: 7.7.1
• joi version: 10.3.1
• environment (node, browser): node
• used with (hapi, standalone, ...): standalone
• any other relevant information:

What are you trying to achieve or the steps to reproduce ?

I try to have a custom error message if a parameter is required conditionally and it is not provided in input. For example for the schema below at least one of the start or match parameters should be provided. So I made start required if match is not provided, and I want to tell user, that one of them should be provided instead of start is required..

const schema = {
  ignore: Joi.array().items(Joi.string()).optional(),
  types: Joi.object().pattern(/\w+/, Joi.array().items(Joi.string())).optional(),
  rules: Joi.array().items(Joi.object().keys({
    type: Joi.string(),
    start: Joi.array().items(Joi.string()).when('match', { is: Joi.empty(), then: Joi.required().error(new Error('CUSTOM MESSAGE')), otherwise: Joi.optional() }),
    match: Joi.alternatives().try(Joi.string(), Joi.object().type(RegExp)).optional(),
  })),
};

const config = {
  ignore: ['space'],
  types:  {
    identifier:  ['select'],
    conjunction: ['and', 'or'],
  },
  rules: [
    { type: 'sil' },
  ],
};

  Joi.assert(config, schema);

Which result you had ?

Exception is thrown for:
TypeError: error.annotate is not a function

What did you expect ?

I expected my custom exception:
CUSTOM MESSAGE