ES6 methods get a null "this" and class variables are inaccessible

I'm using an ES6 class to bundle some functionality together in Node. Here's (basically) what it looks like:

class processDocs {
  constructor(id) {
    this.id = id;
  }

  getDocs(cb) {
    docs
      .query(qb => {
         qb.where('id', this.id);
      })
      .fetch()
      .then(function(documents) {
        cb(null, documents);
      })
    ;
  }

  alterDocs(documents, cb) {
    //some logic
  }

  reindexSearch(cb) {
    //some logic
  }

  process() {
    async.waterfall([
      this.getDocs,
      this.alterDocs,
      this.reindexSearch
    ]);
  }
}


export default processDocs;

I thought that with ES6 classes, the way to assign public variables was to simply reference this and the way to initialize those variables via a constructor is exactly how it shows up in my class definition.

Here's how I'm calling the class (in a separate file):

var Processor = require('./processDocs');

var pr = new Processor(id);
var docs;
pr.process();

Here's the issue, when I console.log out this from the constructor, I get my { id: id } value as predicted; however, whenever I log out this in getDocs when process is running, it's null. BUT, when I log out this in process() right before the waterfall, I get my original object.

Is there any reason for this?

Btw, I'm using node: v0.10.33 and babel-node 4.6.6 and I run babel-node with the --harmony flag. Before anyone asks, I can't update to a newer Node version due to a major dependency which is stuck at v0.10.x.