Investments are collections so it expects a JSON object to be supplied to the attribute as opposed to a singular item. Also, because its a collection if you want to add an investment to an item with an existing ones you will need to include both the existing ones and the ones you want to add. If you don't the existing ones would be removed and only the new ones will be added. I have attached a screenshot as an example.
Release :
Component :
Here is an example: