Unsubscribe from GeoQuery.onSnapshot on document deletion

Question

I have an onWrite Cloud Function

exports.testFunction = functions.firestore.document("test/{docId}").onWrite(async (change, context) => {
    return await test(change);
});

At the topmost of test i have a variable listener, then i have two conditions that checks whether a document was just created or deleted. Inside the first if block i assign ...onSnapshot(...) to listener then inside the second if block i call listener() to unsubscribe from the GeoQuery.onSnapshot subscription made in the first if block, in other words, when a document is created listener is assigned to ...onSnapshot(...). Subsequently when that same document is deleted, i unsubscribe from the previous onSnapshot subscription

const test = async (change) => {
  let listener;

  if (!change.before.exists && change.after.exists) {
    // document created, subscribe
    listener = GeoFirestore.collection(...)
        .where("category", "==", ...)
        .near({ center: ..., radius: ... })
        .onSnapshot(async (snapshot) => {...}, (error) => {...});
  }

  if (change.before.exists && !change.after.exists) {
    // document deleted, unsubscribe
    listener();
  }

The problem is i am getting an error when the second if block gets triggerred

TypeError: listener is not a function

This Cloud Function is in such a way that the first if block will always get called before the second if block (obviously a document can get deleted only after it is created) and according to the Geofirestore docs

onSnapshot

get onSnapshot(): (onNext: (snapshot: GeoQuerySnapshot) =>
void, onError?: (error: Error) => void) => () => void

Defined in src/GeoQuery.ts:74

Attaches a listener for GeoQuerySnapshot events. Returns

(onNext: (snapshot: GeoQuerySnapshot) => void, onError?:
(error: Error) => void) => () => void

An unsubscribe function that can be called to cancel the snapshot listener.

I don't see what i seem to be doing wrong here. I need some help