Qt/C++ - Undefined reference to vtable

Question

I'm getting the dreaded vtable error, but for the love of God I can't figure out what's wrong:

gamemanager.h
#ifndef GAMEMANAGER_H
#define GAMEMANAGER_H

class GameManager : public QObject
{
    Q_OBJECT
public:
    GameManager();
    ~GameManager();
//etc.

};
#endif // GAMEMANAGER_H

And in the .cpp:

gamemanager.cpp:
#include "gamemanager.h"

#include <QTime>

GameManager::GameManager()
{
//etc.
}

GameManager::~GameManager() {    
}

Errors:

gamemanager.cpp:5: error: undefined reference to `vtable for
GameManager' 
gamemanager.cpp:15: error: undefined reference to `vtable
for GameManager'

How can I solve this? Cleaning the project didn't help.