Firebase function (written in NodeJS) to delete file from Cloud Storage when an object is removed from Realtime Database

I am new to NodeJS and I am trying to write the next method for Cloud Functions for Firebase.

What I am trying to achieve:

1. The function should be triggered when the user removes a Photo object from Firebase DB;
2. The code should remove the file object from Storage corresponding to the Photo obj.

These is my Firebase DB structure:

photos/{userUID}/{photoUID}

{
"dateCreated":      "2017-07-27T16:40:31.000000Z",
"isProfilePhoto":   true,
"isSafe":           true,
"uid":              "{photoUID}",
"userUID":          "{userUID}"
}

And the Firebase Storage format:

photos/{userUID}/{photoUID}.png

And the NodeJS code that I am using:

    const functions = require('firebase-functions')
    const googleCloudStorage = require('@google-cloud/storage')({keyFilename: 'firebase_admin_sdk.json' })
    const admin = require('firebase-admin')
    const vision = require('@google-cloud/vision')();

    admin.initializeApp(functions.config().firebase)

    exports.sanitizePhoto = functions.database.ref('photos/{userUID}/{photoUID}')
        .onDelete(event => {

            let photoUID = event.data.key
            let userUID = event.data.ref.parent.key

            console.log(`userUID: ${userUID}, photoUID: ${photoUID}`);

            if (typeof photoUID === 'undefined' || typeof userUID === 'undefined') {
                console.error('Error while sanitize photo, user uid or photo uid are missing');
                return
            }

            console.log(`Deleting photo: ${photoUID}`)

            googleCloudStorage.bucket(`photos/${userUID}/${photoUID}.png`).delete().then(() => {
                console.log(`Successfully deleted photo with UID: ${photoUID}, userUID : ${userUID}`)
            }).catch(err => {
                console.log(`Failed to remove photo, error: ${err}`)
            });

        });

While I run it I get the next error: "ApiError: Not found"

I think these part of the code is the the one that causes the issue:

googleCloudStorage.bucket(`photos/${userUID}/${photoUID}.png`).delete()

Thanks in advance for support and patience.