Error saving core data object

I am quite confused about one error I am receiving when saving an object. I am receiving the following error (when I print out the detailed description):

2015-05-08 08:19:51.589 Br[11240:208443] Core Data Save Error

                           NSValidationErrorKey
activityObject

                           NSValidationErrorPredicate
(null)

                           NSValidationErrorObject
<BRActivity: 0x7deb2aa0> (entity: BRActivity; id: 0x7deb2780 <x-coredata:///BRActivity/t86C0E8CD-2B6C-4AF7-986A-4797B7BEFDF5> ; data: {
activities =     (
);
activityObject = nil;
activityType = 0;
body = "Someone left a comment on your post.";
timestamp = "2015-05-08 04:24:46 +0000";
uuidString = "c6a06b45-e2d5-45bd-9f64-20b13ac87526";
})

                           NSLocalizedDescription
The operation couldn’t be completed. (Cocoa error 1570.)
2015-05-08 08:19:51.590 Br[11240:208443] Core Data Save Error

So from looking on the internet, it seems to be a relationship problem. So a Br post has a relationship called activities relationship, the inverse of which is an activity object. Now that, as we can see from the error, is nil. So what kind of solution are we looking at here... Is there a way to make the relationship "optional" (ok to be nil!) or should I add an activity object? I really don't want to break anything here so if there's a subtle solution let me know. Thanks a bunch guys!

Also here is the method surrounding the save:

 - (void)importArray:(NSArray *)array entityName:(NSString *)entityName attributeName:attributeName error:(NSError *__autoreleasing *)error {
NSParameterAssert(array);
NSParameterAssert(entityName);

[self.context performBlockAndWait:^{
    for (NSDictionary *jsonDictionary in array) {
        NSManagedObject *managedObject = [NSManagedObject upsertWithContext:self.context entityName:entityName dictionary:jsonDictionary attributeName:attributeName error:error];
        if (nil == managedObject) {
            if ([self.context hasChanges]) {
                [self.context rollback];
            }
            return;
        }
    }

    if ([self.context hasChanges]) {
        if (![self.context save:error]) {
            NSError *err = *error;


            NSDictionary *userInfo = [err userInfo];
            if ([userInfo valueForKey:@"NSDetailedErrors"] != nil) {
                // ...and loop through the array, if so.
                NSArray *errors = [userInfo valueForKey:@"NSDetailedErrors"];
                for (NSError *anError in errors) {

                    NSDictionary *subUserInfo = [anError userInfo];
                    subUserInfo = [anError userInfo];
                    // Granted, this indents the NSValidation keys rather a lot
                    // ...but it's a small loss to keep the code more readable.
                    NSLog(@"Core Data Save Error\n\n \
                          NSValidationErrorKey\n%@\n\n \
                          NSValidationErrorPredicate\n%@\n\n \
                          NSValidationErrorObject\n%@\n\n \
                          NSLocalizedDescription\n%@",
                          [subUserInfo valueForKey:@"NSValidationErrorKey"],
                          [subUserInfo valueForKey:@"NSValidationErrorPredicate"],
                          [subUserInfo valueForKey:@"NSValidationErrorObject"],
                          [subUserInfo valueForKey:@"NSLocalizedDescription"]);
                }
            }


            NSLog(@"Error: %@", err.localizedDescription);
        }
        return;
    }
}];
}