co-product without goods receipt

Hello,

We have a Unit, which produce Product and Co-product.

So I have to plan this Co-product in the BOM. After that I create process order which contains this Co-product. But we have two situations:

1. We produce Product and Co-product. It is normal.

2. We produce Product only. So we do goods receipt Product. and doesn't do goods receipt of Co-product. But our module CO says that it wrong. If process order contain Co-product it is obligatory to do doods receipt of Co-product.

What should we do in this situation, because when we create process order we don't know if we will have Co-product or not? It possible to create process order for Co-product, but Controlling says that it is wrong according to costs.

Thank you.