Here's question that I'm trying to solve from time, I even offered a stack overflow bounty but no-one got the problem: I have an entity named Person that may have one home, and an entity named Home that may have one to N homes: Then I have two array controllers: Homes Controller, bound with managed object context, in entity mode: Home; People Controller, bound with managed object context, in entity mode: person. I have a cell based table view, bound with the People Controller.I am able to display successfully the first three columns (name, surname and age), but the problem is with the 4th column: the home's name.Inside the column there is a popup button cell, these are the bindings: Content: Homes Controller.arrangedObjects ; Content values: Homes Controller.arrangedObjects.name; Selected object: People Controller.home. The problem comes at runtime, when I try to click to the popup button to choose the home: Like you see instead of the choice I see "Core data relationship fault", and if I try to change the home by clicking on the menu items, I see a lot of code printed to the console, I post only the most significant line: 2012-11-04 01:47:45.181 Test[4390:303] [<NSManagedObject 0x100156e60> valueForUndefinedKey:]: the entity Home is not key value coding-compliant for the key "(null)". And after this the application freezes. PS: The relationship home and people are the inverse of each other.