SmartFX check out other multiple layers' pixels

I am trying to smartify my plugin. The plugin takes other layers' pixels, selected from a layer param, to put a resized copy of the source layer where the user wants. (Well, that's the gist, anyway). 

Now I am making the plugin Smart FX (everything else is working fine. But I am lost in the business of checking out layers from layer params. 

I seem to be able to do this, no problem: 

extra->cb->checkout_layer(	in_data->effect_ref,
				LAYERPARAM,
				ID,
				&req,
				in_data->current_time,
				in_data->time_step,
				in_data->time_scale,
				&in_result);

I can check in_result, and see my layer. 

But in smartRender when I try to do this...

PF_EffectWorld	*thePixels = NULL;
extra->cb->checkout_layer_pixels(in_data->effect_ref, ID, &thePixels);

...thePixels remains NULL. 

Am I supposed to be doing something with in_result? I cannot see in the examples anything that matches my use case. I had assumed that I only needed the ID to match in pre-render and smart render for the pixels to be obtained.