How to tackle "user": null

[HHuckebein]

Tried to enhance the sample project starting with the user information by doing

    // User Entity
    RKEntityMapping *userEntityMapping = [RKEntityMapping mappingForEntityForName:@"User" inManagedObjectStore:managedObjectStore];
    userEntityMapping.identificationAttributes = @[@"userID"];
    [userEntityMapping addAttributeMappingsFromDictionary:@{
        @"id"          : @"userID",
        @"avatar_url"  : @"avatarURL",
        @"gravatar_id" : @"gravatarID",
        @"url"         : @"jsonURL",
        @"login"       : @"login"}];

    RKResponseDescriptor *userResponseDescriptor = [RKResponseDescriptor responseDescriptorWithMapping:userEntityMapping pathPattern:@"/gists/public" keyPath:@"user" statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];

    [objectManager addResponseDescriptor:userResponseDescriptor];

but as the json contains some users with null values I got an exception in

static NSDictionary *RKEntityIdentificationAttributesForEntityMappingWithRepresentation(RKEntityMapping *entityMapping, NSDictionary *representation)

I can see that in

- (NSArray *)mapRepresentations:(id)representations atKeyPath:(NSString *)keyPath usingMapping:(RKMapping *)mapping

the objectsToMap array some [NSNull null] value are inserted.
Couldn't find any protocol/delegate calls to avoid having null values added to this array.

Any help is appreciated.