I have a method that returns an array:

-(NSArray*)fetchStoresFromContext {
     NSManagedObjectContext *context = [RKManagedObjectStore
                                                        defaultStore].mainQueueManagedObjectContext;
    NSFetchRequest *fetchRequest = [NSFetchRequest
 fetchRequestWithEntityName: @"Store"];
    NSSortDescriptor *descriptor = [NSSortDescriptor sortDescriptorWithKey:
 @"name"ascending:YES];
    fetchRequest.sortDescriptors = @[descriptor];
    NSError *error = nil;
    NSArray *fetchedObjects = [context executeFetchRequest:fetchRequest error:
 &error];
    return fetchedObjects;
}

If create a new NSArray and call this method like this: 

NSArray *test = [self fetchStoresFromContext]; 

everything works fine. [test count] returns 6. 

I have the following property: 

@property (nonatomic, strong) NSArray* stores; 

if I call _stores = [self fetchStoresFromContext] I get the following error: 

-[__NSCFNumber length]: unrecognized selector sent to instance 0x15e3c2f0 

What's going on here? The property is an NSArray and the object is an NSArray as well so why am I having this problem?