RestKit Fail

// MOM has two entities: Entity and ChildEntity. Both have "required" and "optional" string attributes, which are required and optional, respectively. Entity has a to-many relationship to ChildEntity called "children".

NSManagedObjectModel* model = [NSManagedObjectModel mergedModelFromBundles:nil];
store = [[RKManagedObjectStore alloc] initWithManagedObjectModel:model];
[store addInMemoryPersistentStore:NULL];
[store createManagedObjectContexts];

RKEntityMapping* mapping = [RKEntityMapping mappingForEntityForName:@"Entity" inManagedObjectStore:store];

[mapping addAttributeMappingsFromArray:@[@"required", @"optional"]];

RKEntityMapping* childMapping = [RKEntityMapping mappingForEntityForName:@"ChildEntity" inManagedObjectStore:store];

[childMapping addAttributeMappingsFromArray:@[@"required", @"optional"]];
childMapping.discardsInvalidObjectsOnInsert = YES;

[mapping addRelationshipMappingWithSourceKeyPath:@"children" mapping:childMapping];

NSArray* object = @[
                    @{ @"required": @"one",
                       @"optional": @"two",
                       @"children": @[
                               @{@"required": @"four", @"optional": @"three"},
                               @{@"required": @"five", @"optional": @"six"},
                               @{@"optional": @"eight"}
                               ]
                       }
                    ];


NSDictionary *mappingsDictionary = @{ NSNull.null: mapping };
RKMapperOperation *mapper = [[RKMapperOperation alloc] initWithRepresentation:object mappingsDictionary:mappingsDictionary];
mapper.mappingOperationDataSource = [[RKManagedObjectMappingOperationDataSource alloc] initWithManagedObjectContext:store.mainQueueManagedObjectContext cache:store.managedObjectCache];
NSError *mappingError = nil;
BOOL isMapped = [mapper execute:&mappingError];
if (isMapped && !mappingError) {
    // Yay! Mapping finished successfully
    NSError* flub;
    BOOL rslt = [store.mainQueueManagedObjectContext save:&flub];
    NSAssert(rslt == YES, @"Failed to save with error %@",flub);
} else {
    NSLog(@"Mapping error %@",mappingError);
}