I've got a problem processing some footage with COPs

I've got a render that has a few per-light AOVs and I'm making them flash with a CHOP network post render as I need to output various different flashing patterns / sequences without rendering several times.

Everything has been working fine until for one shot I needed overscan and now when I output my network it renders a few - literally 3 or 4 frames - then crashes. All I am doing differently from before when it was working, is comping my overscan render over a constant colour so I can give my client the whole frame as png as that's how they want it.

Does that ring any bells for anyone? I know COPs is buggy but it's also great for some things that no other apps can do...