Getting error on startup. "Error in assignID"

[Oke_oku]

Im making a mod where it adds boxes that can be transported on belts,

At the moment im getting a error on startup saying 'Error in assignID, 'woodenBox' was not recognized id of entity'.
I think the problem is that i haven't made a entity for the box, but i cant find where the entity data is for the copper item to copy off.

Thanks in advance for the help!

[FreeER]

i haven't made a entity for the box

That would be the logical reason for why Factorio isn't finding one

where the entity data is for the copper item

data/base/prototypes/item/demo-item.lua for the copper-ore item (used in recipes), the copper-ore resource entity (that you mine) will be in data/base/prototypes/entity/demo-resources.lua. However, if you are making a box to store things in you should really look in data/base/prototypes/entity/demo-entities.lua for "wooden-chest" or in the entities.lua file at the other 'container' type entities (different types of entities have different requirements, like inventory_size and autoplace definitions).