Unable to deploy Cloud features for Firebase

I performed the functions just fine, but then it stopped working and I don't know why. I'll go back to the sample code ( from here or here ):

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);

// Listens for new messages added to /messages/:pushId/original and creates an
// uppercase version of the message to /messages/:pushId/uppercase
exports.makeUppercase = functions.database.ref('/messages/{pushId}/original')
    .onWrite(event => {
      // Grab the current value of what was written to the Realtime Database.
      const original = event.data.val();
      console.log('Uppercasing', event.params.pushId, original);
      const uppercase = original.toUpperCase();
      // You must return a Promise when performing asynchronous tasks inside a Functions such as
      // writing to the Firebase Realtime Database.
      // Setting an "uppercase" sibling in the Realtime Database returns a Promise.
      return event.data.ref.parent.child('uppercase').set(uppercase);
    });

      

        
        
        
      

    

But now when I run firebase deploy --only functions

I get:

=== Deploying to 'mydb'...

i  deploying functions
i  functions: ensuring necessary APIs are enabled...
i  runtimeconfig: ensuring necessary APIs are enabled...
+  runtimeconfig: all necessary APIs are enabled
+  functions: all necessary APIs are enabled
i  functions: preparing functions directory for uploading...
i  functions: packaged functions (2.04 KB) for uploading
!  functions: Upload Error: Cannot read property 'response' of undefined
i  starting release process (may take several minutes)...
i  functions: updating function makeUppercase...
!  functions[makeUppercase]: Deploy Error: Function load error: Node.js module defined by file index.js is expected to export function named makeUppercase
+  functions: 0 function(s) deployed successfully.


Functions deploy had errors. To continue deploying other features (such as database), run:
    firebase deploy --except functions

Error: Functions did not deploy properly.

      

        
        
        
      

    

What's wrong?

The console shows the same error messages, without any explanation: