CollisionSystem does not name a type

On CollisionSystem m_collisionSystem I have the error "CollisionSystem does not name a type" and I do not know how to handle this. I tried it with forward declaration but m_collisionSystem should be not a pointer.

#pragma once

#include "ui_appwindow.h"
#include "entity.h"
#include "collisionsystem.h"
#include "scene.h"
#include <QMatrix4x4>
#include "iostream"

class Scene;
class Entity;

class Engine {
public:
    Engine(Ui::AppWindow & ui);
    Scene & getScene();
    void start();
    void update(float dt);
    void render();
    std::vector<std::shared_ptr<Entity>> m_entities;

private:
    Ui::AppWindow & m_ui;
    CollisionSystem m_collisionSystem;
};