-[NSCFNumber count]: unrecognized selector

I've got some Core Data code that follows Apple's sample code precisely (the Fetching Attribute Values that Satisfy a Given Function example). I'm using it to get the max value of a field, so I can then increment it when I insert the next object of that entity type. I couldn't get the code to work at all, until I switched my Store Type from `NSXMLStoreType` to `NSSQLiteStoreType`, then all of a sudden everything seemed to be working. However, that's not the case. I noticed that it would always return the same value, even when I inserted objects with a higher one. But, after I quit and reopened (and thus the data was persisted and read back in), it would update with the new inserts. So then I started committing and saving after each insert. After the first "autosave" though, I get the error below (twice in a row): -[NSCFNumber count]: unrecognized selector sent to instance 0x100506a20 This occurs (two times in a rows) when I execute the fetch request once: ``` NSArray *objects = [context executeFetchRequest:request error:&error]; ``` Update I ran my code through the Zombies instrument, and was able to take a look at the object which is getting the error. The call that runs `malloc` to allocate it is: `-[NSUserDefaults(NSUserDefaults) initWithUser:]`. Since I don't have any of my own defaults set, I don't know what object this could be. Update 2 I searched through all of my code for "release" and commented out every `release` or `autorelease` that the static analyzer didn't complain about. I still got the errors. I even went so far as to comment out every last `release`/`autorelease` in my code, and still got it. Now I'm fairly certain my own code isn't over-releasing. Update 3 This post seems to be having the same problem, but his solution doesn't make sense. He changed the result type from `NSDictionaryResultType` to `NSManagedObjectResultType`, which produces an incorrect result. Instead of getting back a single value (the `max` that I'm looking for, that returns back every object of my entity class in the managed object context. Here are the top-most levels of the stack trace (when I have it break on the exception, the first time): ``` #0 0x7fff802e00da in objc_exception_throw #1 0x7fff837d6110 in -[NSObject(NSObject) doesNotRecognizeSelector:] #2 0x7fff8374e91f in ___forwarding___ #3 0x7fff8374aa68 in __forwarding_prep_0___ #4 0x7fff801ef636 in +[_NSPredicateUtilities max:] #5 0x7fff800d4a22 in -[NSFunctionExpression expressionValueWithObject:context:] #6 0x7fff865f2e21 in -[NSMappedObjectStore executeFetchRequest:withContext:] #7 0x7fff865f2580 in -[NSMappedObjectStore executeRequest:withContext:] ``` I've seen this question on numerous forums elsewhere on the web, but no one has offered a workable solution. By popular request, I added my own code below. To explain slightly, my entity's name is `Box` and the property I'm trying to get the value of is "sortOrder", an `Int 32` attribute. ``` NSManagedObjectContext *context = [MyLibrary managedObjectContext]; NSFetchRequest *request = [[NSFetchRequest alloc] init]; [request setEntity:[NSEntityDescription entityForName:@"Box" inManagedObjectContext:context]]; // Specify that the request should return dictionaries. [request setResultType:NSDictionaryResultType]; // Create an expression for the key path. NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"sortOrder"]; // Create an expression to represent the function you want to apply NSExpression *expression = [NSExpression expressionForFunction:@"max:" arguments:[NSArray arrayWithObject:keyPathExpression]]; // Create an expression description using the minExpression and returning a date. NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init]; // The name is the key that will be used in the dictionary for the return value. [expressionDescription setName:@"maxSort"]; [expressionDescription setExpression:expression]; [expressionDescription setExpressionResultType:NSInteger32AttributeType]; // Set the request's properties to fetch just the property represented by the expressions. [request setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]]; // Execute the fetch. NSError *error; NSNumber *requestedValue = nil; NSArray *objects = [context executeFetchRequest:request error:&error]; NSLog( @"objects: %@", objects ); if (objects != nil && [objects count] > 0) { requestedValue = [[objects objectAtIndex:0] valueForKey:@"maxSort"]; } else { [[NSApplication sharedApplication] presentError:error]; } [expressionDescription release]; [request release]; NSLog( @"Max Sort Order: %@", requestedValue ); return requestedValue; ```