dataloader - async map issue

.babelrc

{
  "presets": ["es2015"]
}

index.js

import DataLoader from 'dataloader';
import redis from 'redis';
import express from 'express';

class RedisMap {
  constructor() {
    this.client = redis.createClient();
  }
  get(key) {
    // return false;
    return new Promise((resolve, reject) => {
      this.client.get(key, value => {
        if (value) {
          return resolve(value);
        }

        return resolve(undefined);
      });
    });
  }
  set(key, valuePromise) {
    valuePromise.then(value => {
      this.client.set(key, value);
    });
  }
  delete(key) {
    this.client.del(key);
  }
  clear() {
    this.client.flushdb();
  }
}

const loader = new DataLoader(
  keys => Promise.resolve(keys.map(key => `${key} loaded`)),
  {
    cacheMap: new RedisMap()
  }
);

const app = express();

app.get('/:id', (req, res) => {
  loader.load(req.params.id).then(val => {
    res.send(val);  
  });
});

app.listen(3000, () => {
  console.log('running √');
});

package.json

{
  "dependencies": {
    "babel-cli": "^6.16.0",
    "babel-preset-es2015": "^6.16.0",
    "dataloader": "^1.2.0",
    "express": "^4.14.0",
    "redis": "^2.6.2"
  }
}