android – error in logcat “Cannot convert object of sort java.lang.String to sort”

I’m attempting to retrieve information from Firebase underneath uid. I get an error logcat

Cannot convert object of sort java.lang.String to sort

Codes:

    recview = (RecyclerView) findViewById(R.id.recview);
    recview.setLayoutManager(new LinearLayoutManager(this));

    String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
    DatabaseReference db = FirebaseDatabase.getInstance().getReference();
    DatabaseReference uidRef = db.little one("Customers").little one(uid);

    FirebaseRecyclerOptions<mannequin> choices =
            new FirebaseRecyclerOptions.Builder<mannequin>()
                    .setQuery(uidRef, mannequin.class)
                    .construct();

    adapter=new myadapter(choices);
    recview.setAdapter(adapter);

That is db.i’m attempting to retreive filename

    {
  "Customers": {
    "NseKqjViW4XO2RshRGQ7kW1swbv2": {
      "-NE4twoPdj2ZiypGd2D-": {
        "filename": "Math"
      },
      "-NE57EGP7DbgRll3yvlJ": {
        "filename": "Science"
      }
    }
  }
}