MongoSkin - using findOne() and update() together results in UnhandledPromiseRejectionWarning

Question

I am using Mongoskin together with Javascript and have achieved some great results so far, until I got issues with combining two different parts of it.

When using findOne() with update(), I get an error message I am not sure about. I think there might be something wrong with how I am handling the callback. When using findOne I can see the results from it, but when passing it down to the update-function it fails. Since the error message mentions it needs a valid Javascript object, my first thought is that the update-function is called too early or something...

db.collection('colname').findOne({}, function (err, result) {
  db.collection('colname').update({
    $set: {
      "thirdArray.3": result.firstArray[7],
      "fourthArray.3": result.secondArray[7]
    }
  });
});

and the error message:

UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1):
MongoError: document must be a valid JavaScript object