Core Data: Error if deleting unsaved object

I have some trouble with a core data project. I have a NSArrayController with NSManagedObjects in it, and + / - buttons. If i delete a object in the row after the file has been saved it works perfectly, but if I add a new object, and immediately delete it again (without changing any of the default values), i get an error:

Serious application error.  Exception was caught during Core Data change processing: Unknown number type or nil passed to arithm开发者_如何学JAVAetic function expression. with userInfo (null)  
Unknown number type or nil passed to arithmetic function expression.  

It's the same case if i undo the add of the new object.

Anybody able to give me a pointer what to do? I assume it has something to do with that it only has an temporary ID, but how to solve it i don't know.

I found the problem and solution. For the record i will describe it.

The problem was that the object in its init function added an observer on itself. This was probably what triggered the error. If i before i delete the object remove itself as observer, the error is not produced.

Tables don't usually contain NSManagedObjects. Rather, either an NSArrayController is providing the managed objects' attributes as values via bindings or an object conforming NSTableViewDataSource is. The deletion of a selected object provided in either of these scenarios can be done a myriad of ways. The problem is, you haven't explained at all how your case is set up.

Is it possible your deletion (whatever target/action your button is wired to) is being passed nil or something unexpected? I can imagine a similarly-frighenting error if you asked a managed object context to delete a float value or some other random thing ...