Programmatically created UICollectionView: cells overlap after scrolling

I'm trying to implement a custom UICollectionView that shows all elements from the asset library. While displaying the items is not that big of a deal, I run into a weird bug which I think has to do something with the layout I'm using.

Let's start with some screenshots to indicate what's going on. I'll just post the links to these images to be easy on your scroll wheels :-)

1. Initial

https://dl.dropboxusercontent.com/u/607872/stackoverflow/01-uicollectionview-start.png

Everything ok.

2. Scrolled down

https://dl.dropboxusercontent.com/u/607872/stackoverflow/02-uicollectionview-scrolleddown.png

The images from the first row (which isn't visible anymore) repeat behind the images in the last row. The first one is invisible because it is entirely overlapped.

3. Scrolled up again

https://dl.dropboxusercontent.com/u/607872/stackoverflow/03-uicollectionview-scrollup.png

The first image (in the background) is from the same row, but should be the third. The 2nd and 3rd image are from the last row.

Here's some code. I hope I didn't miss any relevant stuff - if so, please let me know.

MultiImagePickerController.m (:UICollectionViewController, protocols: UICollectionViewDataSource, UICollectionViewDelegateFlowLayout)

- (void)loadView {
    [...] // layout stuff
    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
    [layout setScrollDirection:UICollectionViewScrollDirectionVertical];

    _collectionView = [[UICollectionView alloc] initWithFrame:collectionRect collectionViewLayout:layout];
    [_collectionView setDataSource:self];
    [_collectionView setDelegate:self];
    [_collectionView registerClass:[MultiImagePickerCell class] forCellWithReuseIdentifier:@"MultiImagePickerCellIdentifier"];
    [_collectionView setBackgroundColor:[UIColor whiteColor]];
    [self.view addSubview:_collectionView];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    MultiImagePickerCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"MultiImagePickerCellIdentifier" forIndexPath:indexPath];

    if ( !cell ) {
        CGSize sizeCell = [MultiImagePickerCell defaultSize];
        cell = [[MultiImagePickerCell alloc] initWithFrame:CGRectMake( 0, 0, sizeCell.width, sizeCell.height )];
    }

    NSString *groupname = [self getKeyFromMutableDictionary:assetList forIndex:indexPath.section];
    [cell addImageUsingAsset:[[assetList objectForKey:groupname] objectAtIndex:indexPath.row]];

    return cell;
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath {
    CGSize sizeCell = [MultiImagePickerCell defaultSize];
    return CGSizeMake( sizeCell.width + 10.0, sizeCell.height + 10.0 );
}

MultiImagePickerCell - (void)addImageUsingAsset:(ALAsset *)asset;

This method adds a UIImageView and a UIImage to the cell to show the asset. Nothing special, in my opinion. When I set breakpoints in cellForItemAtIndexPath, I can see that the correct asset is being read from the assetList and the views are calculated correctly. The drawRect method of the cell is NOT implemented.

I think I experienced the same problem a few years back when I tried to programmatically create a UITableViewController, but I can't remember how I solved it.

Any suggestions?