Article ... has a missing container.

[Jujutsuka]

Does anyone know what this means? I have exported articles from my mechanics database, which have no container. They are containers for other articles. The 'containing article' view on the right pane states "-no container-" which is correct, because there is and should be no container. Why am I being told upon import that the article is missing a container, when there shouldn't be one to begin with.