https://github.com/sideway/joi/blob/ec2381ecab85d2789333618dd799b8ee14bc0aca/lib/index.d.ts#L626 ValidationError._object needs to be ValidationEror._original. See: errors.js exports.ValidationError = class extends Error { constructor(message, details, original) { super(message); **this._original = original;** this.details = details; } static isError(err) { return err instanceof exports.ValidationError; } };