i am trying to update a subcollection field, which is in an array, but cannot get with where(). wha am i doing wrong?

  async updateDialogueStatus({ state }, data){
    const whoDis =  this.$fire.auth.currentUser
    if (data.dialogue.length > 0) {
      const lastDialogueInArray = data.dialogue.slice(-1)[0];

      const db = this.$fire.firestore;
      db.collection('conversations')
      .doc(data.id)
      .collection('dialogue')
      .where("messageId", "==", lastDialogueInArray.messageId)
      .update({
        status: "read",
        readBy: whoDis.email
      })
    } else {

i am getting this error: Uncaught (in promise) TypeError: db.collection(...).doc(...).collection(...).where(...).update is not a function