readDocument Google Cloud Vision in Cloud Functions does not work

I need to write a Cloud Function, and the sole permitted programming language is Node. Honestly I just have a slight idea about Node, but I am daring to write code based on some examples given in the GCP documentation. Here it is:

'use strict';

// [START functions_ocr_setup]
const config = require('./config.json');

// Get a reference to the Cloud Storage component
const storage = require('@google-cloud/storage')({
 keyFilename: './pathtomyserviceaccount.json',
 projectId: 'projectId'
});

// Get a reference to the Cloud Vision API component
const vision = require('@google-cloud/vision')({
  keyFilename: './pathtomyserviceaccount.json',
  projectId: 'projectId'
});

// [END functions_ocr_setup]

// [START functions_ocr_detect]

function detectText (file) {

  console.log('Haya vamos!');

  return vision.readDocument(file)

    .then((results) => {
        const fullTextAnnotation = results[1].responses[0].fullTextAnnotation;
        console.log(fullTextAnnotation.text);
    })
    .catch((err) => {
        console.error('ERRORes:', err);
    });

}

// [END functions_ocr_detect]

// [START functions_ocr_process]
/**
 * Cloud Function triggered by Cloud Storage when a file is uploaded.
 *
 * @param {object} event The Cloud Functions event.
 * @param {object} event.data A Google Cloud Storage File object.
 */
exports.processImage = function processImage (event) {

    let file = event.data;

    return Promise.resolve()

    .then(() => {
      if (file.resourceState === 'not_exists') {
        // This was a deletion event, we don't want to process this
        return;
      }

      if (!file.bucket) {
        throw new Error('Bucket not provided. Make sure you have a "bucket" property in your request');
      }
      if (!file.name) {
        throw new Error('Filename not provided. Make sure you have a "name" property in your request');
      }

      return detectText(storage.bucket(file.bucket).file(file.name));
    })
    .then(() => {
      console.log(`File ${file.name} processed.`);
    });

};
// [END functions_ocr_process]

The file 'serviceaccount.json' is located in the same folder as the index.js (yes, it contains the function 'processImage' that I want GC to execute).

Looking in the log of the corresponding GC Funtion, I get the following error:

PartialFailureError: A failure occurred during this request

I've been doing some testing, and I intuit that the error is related to the 'readDocument' call... Once again, I recognize I'm a newcomer in Node, so I would really appreciate any hint or suggestion!!! Many thanks in advance!!!

via Rafael Vizcaino Gonzalez