Firebase Function local debug doesn't work

Question

I am currently trying to debug my firebase functions to trigger notifications and I found out that i can do that with firebase cli by using this command

firebase emulators:start 

For some reason, when I call my function on the localhost:5001 I get this error

Cannot read properties of undefined (reading 'toString')

and in the terminal I get this

!  functions: TypeError: Cannot read properties of undefined (reading 'toString')
    at C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\lib\emulator\functionsEmulatorRuntime.js:688:54
    at Layer.handle [as handle_request] (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:149:13)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:145:7)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:145:7)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:145:7)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:145:7)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:145:7)
    at next (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:145:7)
    at Route.dispatch (C:\Users\mrxbu\AppData\Roaming\npm\node_modules\firebase-tools\node_modules\express\lib\router\route.js:119:3)
!  Your function was killed because it raised an unhandled error.

Am I doing something wrong?

Function:

const functions = require("firebase-functions");
const admin = require("firebase-admin");

admin.initializeApp({
  credential: admin.credential.applicationDefault(),
  databaseURL: "https://console.firebase.google.com/project/projectname",
});

exports.sendNotificationOnFighterDataChangeToFans = functions.firestore
    .document("users/{userId}")
    .onUpdate(async (change, context) => {
      const before = change.before.data(); // Data before the change
      const after = change.after.data(); // Data after the change

      // Check if userName changed
      if (before.weightClass !== after.weightClass) {
        const userId = context.params.userId;

        const fighterSnapshot = await admin
            .firestore()
            .collection("users")
            .doc(userId)
            .get();

        const fighterData = fighterSnapshot.data();

        if (fighterData && fighterData.route === "fighter") {
          const followers = fighterData.followers || [];

          const fanPromises = followers.map(async (followerId) => {
            const fanSnapshot = await admin
                .firestore()
                .collection("users")
                .doc(followerId)
                .get();

            const fanData = fanSnapshot.data();
            if (fanData && fanData.route === "fan" && fanData.deviceToken) {
              const notification = {
                token: fanData.deviceToken,
                notification: {
                  title: "UserName Changed",
                  body: `Your userName has been changed to ${after.userName}`,
                },
              };

              try {
                await admin.messaging().send(notification);
                console.log("Notification sent successfully");
              } catch (error) {
                console.error("Error sending notification:", error);
              }
            }
          });

          await Promise.all(fanPromises);
        }
      }
    });

localhost url structure

http://localhost:5001/projectname-83750/us-central1/sendNotificationOnFighterDataChangeToFans-0