Kryo Serializer

Question

I Have some java code that currently works like this:

Client:
...
serializer.serialize(new ClientGetAllRequest(keys));

Server:
Object request = serializer.getObject();
...

Where the ClientGetAllRequest Class is just a class with a field List<String> keys (Similarly for PutAll but it's a Map<String,byte[]> not a List). The serializer works like this:

public class KryoSerializer {
    private Output output;
    private Input input;
    private Kryo kryo;

    public KryoSerializer() {
        kryo = new Kryo();
        kryo.register(byte[].class);
        kryo.register(ArrayList.class);
        kryo.register(HashMap.class);
        kryo.register(HashSet.class);
        kryo.register(ClientRequest.class);
        kryo.register(ClientError.class);
        kryo.register(ClientGetAllRequest.class);
        kryo.register(ClientPutAllRequest.class);
        kryo.register(ClientPutAllResponse.class);
        kryo.register(ClientGetAllResponse.class);
        //(Here actually many other classes are registered for another part of the code irrelevant to this question)
        kryo.setRegistrationRequired(true);
    }

    public void setInputStream(InputStream inputStream) {
        this.input = new Input(inputStream, Config.getConfig().max_object_size);
    }

    public void setOutputStream(OutputStream outputStream) {
        this.output = new Output(outputStream, Config.getConfig().max_object_size);
    }

    public void serialize(Object object) throws IOException {
        kryo.writeClassAndObject(this.output, object);
        this.output.flush();
        this.output.getOutputStream().flush();
    }

    public Object getObject() {
        return kryo.readClassAndObject(input);
    }
}

I want to add a field String client_thread_id to the request but every time i try i get a serializing error, where for some reason the serializer at the server side tries to read one of the keys in the list as a class:

com.esotericsoftware.kryo.KryoException: Unable to find class: user1512450

I'm not sure what I'm doing wrong but I'm not very familiar on how to work with Kryo Serializers. Does anyone know where I should be looking to solve this error?