UIDocument loadFromContents should return a NSFileWrapper; returns NSConcreteData instead

I am making a text editor app that stores each of its document as a NSFileWrapper directory, with document text and document title as separate files in the directory. I expect the contents part of loadFromContents: (id) contents to be a NSFileWrapper, however this is not the case. My code is below (it belongs to a subclass of UIDocument):

// loads document data to application data model

- (BOOL) loadFromContents:(id)contents ofType:(NSString *)typeName error:(NSError *__autoreleasing *)outError {
    // from the contents, extract it so that we have the properties initialized
    self.fileWrapper = (NSFileWrapper *) contents;
    NSLog(@"%@", [contents class]); //** returns NSConcreteData

    // get the fileWrapper's children
    NSDictionary *contentsOfFileWrapper = [contents fileWrappers];

    // assign things to the document!
    // can also be done lazily through getters

    self.text = [contentsOfFileWrapper objectForKey:TEXT_KEY];
    self.title = [contentsOfFileWrapper objectForKey:TITLE_KEY];
    if ([self.delegate respondsToSelector:@selector(noteDocumentContentsUpdated:)]){
        [self.delegate noteDocumentContentsUpdated:self];
    }
    return YES;
}

When I tried to call this method, I get this error:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSConcreteData fileWrappers]: unrecognized selector sent to instance 0x9367150'

Below is my contentsForType: function if it helps:

- (id) contentsForType:(NSString *)typeName error:(NSError *__autoreleasing *)outError {
    if (!self.fileWrapper) {
        self.fileWrapper = [[NSFileWrapper alloc] initDirectoryWithFileWrappers:nil];
    }

    NSDictionary *childrenFileWrappers = [self.fileWrapper fileWrappers];

    // now if we have a text but it is not represented, in the file wrapper, put it in. Same with the images.
    if ([childrenFileWrappers objectForKey:TEXT_KEY] == nil && self.text != nil) {
        NSData *textData = [self.text dataUsingEncoding:kCFStringEncodingUTF16];
        NSFileWrapper *textFileWrapper = [[NSFileWrapper alloc] initRegularFileWithContents:textData];
        [textFileWrapper setPreferredFilename:TEXT_KEY];
        [self.fileWrapper addFileWrapper:textFileWrapper];
    }

    if ([childrenFileWrappers objectForKey:TITLE_KEY] == nil && self.title != nil) {
        NSData *titleData = [self.title dataUsingEncoding:kCFStringEncodingUTF16];
        NSFileWrapper *titleFileWrapper = [[NSFileWrapper alloc] initRegularFileWithContents:titleData];
        [titleFileWrapper setPreferredFilename:TITLE_KEY];
        [self.fileWrapper addFileWrapper:titleFileWrapper];
    }

    return self.fileWrapper;

}

Thanks!

I fixed the problem. There was a .DS_Store file in my Documents folder and it messed up the results. Once I added an if statement to rule it out everything worke fine.