Firebase cloud function to send notification to multiple topics (.sendMulticast() error)

I’ve tried to make a cloud function that sends a notification to multiple topics. However when I use .sendMulticast() I receive error “Error: tokens must be a non-empty array
at Messaging.sendMulticast”.I don’t want to have any tokens since I have already specified the topics I’m sending the notification to. Here is my current cloud function:

exports.newStoreTopic = functions
    .region("europe-west2")
    .firestore
    .document("StoreDatabase/{productName}")
    .onCreate(async (snapshot, context) => {
      const newStore = snapshot.data();
      const store = newStore.StoreName;
      const message = {
        notification: {
          title: "New Store!",
          body: store+" has just been added",
        },
        topic: ["storeUpdates", newStore.StoreName],
      };
      return admin
          .messaging()
          .sendMulticast({
            tokens: [],
            topic: ["storeUpdates", store],
            message,
          })
          .then(() => {
            console.log("Notification sent successfully");
            return null;
          })
          .catch((error) => {
            console.error("Error sending notification:", error);
            return null;
          });
    });