Unsupported configuration Multiple objects using identifier "" (Identifiers must be unique)

Question

I just started using array controllers, so my experience with them is very limited.

I get this warning in Xcode 4.1:

Unsupported configuration Multiple objects using identifier "" (Identifiers must be unique)

There is nothing inside double quotes. I am using an array controller. It is displayed for each column of the table. I checked the bindings for each column. Each column is associated with a unique identifier [i.e. arrangedObjects.fullName].

Everything is working fine, and I think I can just ignore it.