"TypeError: module is undefined" when combining all of my modules into a minified js file

Question

I have several .js files where each one holds exactly one module and each module has the following pattern (this is a simplified example):

"use strict";
/*jshint esversion: 6 */
/*jshint node: true */
/*jshint unused:false*/
/*globals globalVariable, someModule, $:false */
var aModule = (function () {
   let variable = 100;
   return {
      aFunction: function (){
         //
      }
   };
})();

When I use wro4j to minify and merge all the js files I get an error in firefox as in the title.

This is how a parsed module looks in the minified file:

"use strict";var aModule=(function(){...}};})();"use strict";var anotherModule...

This error happens with any first usage of a module in my application, whether from the main jsp or from a call in a module to another module.

Do you think it has anything to do with an ES6 feature that the current version of wro4j still cannot handle correctly?

Let me know if I should update with more info, thanks.