Not triggering React event correctly

I have a Reflux store which seems to cause an error when triggering an action:

var taskStore = Reflux.createStore({
  listenables: [taskActions],
  init: function(){
    this.tasks = [];
    this.index = {};
    this.trigger(this.tasks);
  },
  onLoad: function(arr){
    var idx = {}
    arr.forEach(function(ele){
      idx[ele.id] = ele
    });
    this.tasks = arr;
    this.index = idx;
    this.trigger(arr);
  }
});

When the action load is called and the onLoad method triggers, it fails on the this.trigger method with Chrome saying the error is Uncaught TypeError: Cannot read property 'apply' of undefined

The call stack eventually gets me to the Reflux listen method:

listen: function(callback, bindContext) {
    bindContext = bindContext || this;
    var eventHandler = function(args) {
        callback.apply(bindContext, args);
    }, me = this;
    this.emitter.addListener(this.eventLabel, eventHandler);
    return function() {
        me.emitter.removeListener(me.eventLabel, eventHandler);
    };
},

It's the callback in callback.apply, but it looks like that callback is undefined for some reason .

I'm clearly doing something wrong to trigger such a deep error, but I don't know what. Can someone tell me what it is that I'm doing wrong in my store?