Spawning An Actor Whithin Another Results in Crash

Development Programming & Scripting C++
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1

I have the following classes:

Hand.h:

  #pragma once
  #include "GameFramework/Actor.h"
  #include "Hand.generated.h"
    
    /**
     * 
     */

    UCLASS(Within = Arm)
    class AHand : public AActor
    {
            //code
    };

Arm.h:

#pragma once
    
    
    #include "GameFramework/Actor.h"
    
    #include "Hand.h"
    #include "Arm.generated.h"
    
    UCLASS()
    class AArm : public AActor
    {
    	GENERATED_UCLASS_BODY()
    
    
            void createHand();
    
    	UPROPERTY()
    		AHand * handPart;


   };

Arm.Cpp:

void AArm::createHand()
{
     FActorSpawnParameters spawnInfo;
     spawnInfo.Owner = this;

     handPart = GetWorld()->SpawnActor<AHand>(spawnInfo);
}

This compiles fine, but when I run AArm::createHand() during runtime the game crashes. I get this error message in the Log:

[2014.07.21-03.11.48:269][638]LogWindows: === Critical error: === Unhandled Exception: 0x00000001
[2014.07.21-03.11.48:270][638]LogWindows: Object Hand Hand created in Level instead of Arm

If I change UCLASS(Within = Arm) to UCLASS() in Hand.h, no crash occurs.

Why am I crashing and how do I fix it?
Thanks!