"Attempting to use the forward class 'Game' as superclass of 'MathGame'" in Cocos2d

I'm making a Cocos2d game for iphone, and I have my main game mode, Game, which inherits from CCLayer.

I'm trying to make another game mode, MathGame, which inherits from Game, but when I try to compile, I get this error in MathGame.h:

Attempting to use the forward class 'Game' as superclass of 'MathGame'

I get the error even if the implementation and interface of MathGame are empty. And it only happens if I try to include MathGame.h in another file.

Here's the code for the Game class:

// Game.h
#import "cocos2d.h"
#import <GameKit/GameKit.h>
#import "SplashScreenLayer.h"

@interface Game : CCLayer
    // A bunch of stuff
@end

The new game type:

// MathGame.h
#import "Game.h"

@interface MathGame : Game
@end

And the main menu that includes both:

// SplashScreen.h
#import "cocos2d.h"
#import "Game.h"
#import "MathGame.h"
#import "HowToPlayLayer.h"
#import "AboutLayer.h"

@interface SplashScreenLayer : CCLayer
    // A bunch of stuff
@end

I can't find anything helpful online. Any ideas?