Account Assignment Category empty for stock...

Hi all,

We are doing an upgrade to ecc 6.0 from 4.7, and in 4.7 we have the account assignment called stock that the id is the field empty (without any value).

I've created the same field (empty for stock) in 6.0 and the problem is that I'm not hable to use it because it says that I have to fill the account assignment.

Is this normal? Am I hable to use empty? How?