STRING is casting empty string to 'true'

When I intentionally provide empty value to option, Caporal turns it into 'true', I resolved it using custom validator using following

  static isString(val) {
    if (typeof val !== 'string') {
      throw new Error('invalid type');
    }
    return val;
  }

When I provide a number, it fails so that is all good. but the error message contains


error: invalid type

<path>/node_modules/@caporal/core/dist/index.js:1
module.exports=function(t){var e={};function r(n){if(e[n])return e[n].exports;var o=e[n]={i:n,l:!1,exports:{}};return t[n].call(o.exports,o,o.exports,r),o.l=!0,o.exports}return r.m=t,r.c=e,r.d=function(t,e,n){r.o(t,e)||Object.defineProperty(t,e,{enumerable:!0,get:n})},r.r=function(t){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(t,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(t,"__esModule",{value:!0})},r.t=function(t,e){if(1&e&&(t=r(t)),8&e)return t;if(4&e&&"object"==typeof t&&t&&t.__esModule)return t;var n=Object.create(null);if(r.r(n),Object.defineProperty(n,"default",{enumerable:!0,value:t}),2&e&&"string"!=typeof t)for(var o in t)r.d(n,o,function(e){return t[e]}.bind(null,o));return n},r.n=function(t){var e=t&&t.__esModule?function(){return t.default}:function(){return t};return r.d(e,"a",e),e},r.o=function(t,e){return Object.prototype.hasOwnProperty.call(t,e)},r.p="",r(r.s=29)}([function(t,e){t.exports=require("chalk")},function(t,e){t.exports=require("path")},function(t,e){t.exports=require("util")},function(t,e){t.exports=require("lodash/filter")},function(t,e){t.exports=require("lodash/flatMap")},function.....

fe: invalid type
    at t (<path>/node_modules/@caporal/core/dist/index.js:1:21469)
    at ye (<path>/node_modules/@caporal/core/dist/index.js:1:21518)
    at <path>/node_modules/@caporal/core/dist/index.js:1:23317
    at <path>/node_modules/@caporal/core/dist/index.js:1:23339
    at <path>/node_modules/@caporal/core/node_modules/lodash/_baseReduce.js:18:9
    at <path>/node_modules/@caporal/core/node_modules/lodash/_createBaseFor.js:17:11
    at baseForOwn (<path>/node_modules/@caporal/core/node_modules/lodash/_baseForOwn.js:13:20)
    at <path>/node_modules/@caporal/core/node_modules/lodash/_createBaseEach.js:17:14
    at baseReduce (<path>/node_modules/@caporal/core/node_modules/lodash/_baseReduce.js:15:3)
    at reduce (<path>/node_modules/@caporal/core/node_modules/lodash/reduce.js:48:10) {
  meta: {}
}

it prints entire /node_modules/@caporal/core/dist/index.js file and then the error trace. Any clues on what's going on?

mattallty / Caporal.js

STRING is casting empty string to 'true' #214