.default is not a constructor with ES2015 Import, Export, browserify and class

Hi - short question for you.

• In my Mouse.js I export a class "Mouse".

export class Mouse {
    constructor(options){
        let defaultOptions = {
            dragLength: 0,
        };
        this.options = mergeObjects(defaultOptions,options);

        this._startX = 0;
        this._x = 0;
        this._isDown = false;
        this._isDragging = false;
    }

    isBetween(left, right){
        return (this._x >= left) && (right >= this._x);
    }
}

• In my Schedule.js I import this class with import Mouse from './Mouse'; I then use it like this:

class Schedule {
   constructor(options) {
   this.options = options;
   this.mouse = new Mouse();
 }
...
}

• I get the following error:

Schedule.js: Uncaught TypeError: _Mouse2.default is not a constructor

Any idea guys? Thank you very much!