Firebase -- Document references must have an even number of segments, but Rezepte has 1

Ok so I generated a random name of a document and now I'm trying to get data from that document.

        namereference
                .get()
                .addOnSuccessListener(new OnSuccessListener<DocumentSnapshot>() {
                    @Override
                    public void onSuccess(DocumentSnapshot documentSnapshot) {
                        if (documentSnapshot.exists()){
                            String names = documentSnapshot.get("dokumentnamen").toString().replace("[", "").replace("]", "");
                            String [] namelist = names.split(", ");
                            Random random = new Random();
                            int randomNumber = random.nextInt(namelist.length);
                            randomrezeptname = namelist[randomNumber];

                        }
                    }
                });
    } 

Generating a random name works fine and the name exists.
But now I'm trying to use that String in a DocumentReference within another void but it doesn't work at all.

DocumentReference documentReference = db.collection("Rezepte").document(this.randomrezeptname);

I always get the same error:
>Invalid document reference. Document references must have an even number of segments, but Rezepte has 1

I tried using the content of the String and put it directly in the DocumentReference and it worked. But I have to use a variable since It's supposed to show a random document.
Any ideas what I did wrong?

I'm guessing that when you say
> I tried using the content of the String and put it directly in the DocumentReference and it worked

You mean that you ran

DocumentReference documentReference = db.collection("Rezepte").document("a_random_recipe_name");

And it worked, suggesting that your hierarchy is fine.

Are you sure that the random name generator thing actually sets randomrezeptname before you try to use it to access a random document? I mean to say that if you run:

1) namereference.get().addOnSuccessListener(OnSuccessListener);
2) System.out.println(this.randomrezeptname)
3) DocumentReference documentReference = db.collection("Rezepte").document(this.randomrezeptname)

Lines 2 and 3 will probably run before OnSuccessListener is run, meaning you'll have some odd behavior around whatever randomrezeptname is before OnSuccessListener sets it. OnSuccessListener runs some time later, on the main thread, but after an asynchronous method returns.