Instanced Static Mesh Component: Crash upon adding instances in blueprint

Development Asset Creation
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1

I’ve made a new class deriving from AActor C++, which holds both, a static mesh component, and an instanced static mesh component:

protected:
UPROPERTY(VisibleAnywhere, Category="cat")
	UStaticMeshComponent * sm;
UPROPERTY(VisibleAnywhere, Category="cat")
    UInstancedStaticMeshComponent * ism;

They are initialized in the constructor as follows:

sm = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("Locomotive"));
SetRootComponent(sm);
ism = CreateDefaultSubobject<UInstancedStaticMeshComponent>(TEXT("Wagons"));
ism->SetupAttachment(sm);

When I now try to create a blueprint of the Actor, I cannot add any instances to the instanced static mesh in blueprint editor, as the engine crashes with:

checkf(Stream.VertexBuffer->IsInitialized(), TEXT("Vertex buffer was not initialized! Stream %u, Stride %u, Name %s"), StreamIndex, Stream.Stride, *Stream.VertexBuffer->GetFriendlyName());

(in VertexFactory.cpp:184)

If I however add the instanced static mesh component via blueprint, everything works. I can also add instances to the instanced static mesh if it’s placed in a level instead of adding the instances in the blueprint editor.
Am I missing some initialization step for the instanced static mesh component?