I am getting an Firebase Error: "Unknown field filter op"

Does anyone know what the error "FirebaseError: Unknown field filter op" means?

I working on a Vue project where I store playlists in a Firestore Database, and I want to make CRUD operations.
This error pops up when I try to recieve a single document from the database.
I am not sure where to look for the error.

<template>
    <div v-if="playlist">
        <h2> {{ playlist.title }} </h2>
    </div>
</template>

<script>

import db from '../firebase/config'
export default {

  data() {
      return {
          playlist: null
      }
  },
  created(){
      let ref = db.collection('playlists').where('slug', '=', this.$route.params.playlist_slug)
      ref.get().then(snapshot => {
          snapshot.forEach(doc => {
              this.playlist = doc.data()
              this.playlist.id = doc.id
          })
      })
  }
 
}
</script>

<style scoped>

</style>