IPhone : Why is this UIImage and UILabel collapsed?

I'm trying to put a UIImage + UILabel in a UICollectionViewCell.

If I'm only to use a background color in that UIImage, it works perfectlly:

When I switch to a namedImage, I got this:

As you can see, the label become invisible somehow, I couldn't tell why.

Attached code:

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *identifier = @"Cell";

    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];
    UIImageView *imageView = [[UIImageView alloc] init];
    [imageView setImage: [UIImage imageNamed:@"avatar"]];
//    [imageView setBackgroundColor: [UIColor yellowColor]];
    imageView.contentMode = UIViewContentModeScaleAspectFit;
    imageView.translatesAutoresizingMaskIntoConstraints = NO;

    UILabel *label = [[UILabel alloc] init];
    [label setTextAlignment: NSTextAlignmentCenter];
    [label setText:@"aaaa"];
    [label setTextColor: [UIColor whiteColor]];
    [label setBackgroundColor: [UIColor redColor]];
    label.translatesAutoresizingMaskIntoConstraints = NO;

    [cell addSubview: label];
    [cell addSubview: imageView];
    [cell sendSubviewToBack: imageView];

    [cell addConstraints:[NSLayoutConstraint
                                constraintsWithVisualFormat:@"H:|-0-[imageView]-0-|"
                                options:NSLayoutFormatAlignAllBaseline metrics:0
                                views:NSDictionaryOfVariableBindings(imageView, label)]];
    [cell addConstraints:[NSLayoutConstraint
                          constraintsWithVisualFormat:@"H:|-0-[label]-0-|"
                          options:NSLayoutFormatAlignAllBaseline metrics:0
                          views:NSDictionaryOfVariableBindings(imageView, label)]];
    [cell addConstraints:[NSLayoutConstraint
                          constraintsWithVisualFormat:@"V:|-0-[imageView]-30-[label]-0-|"
                          options:NSLayoutFormatAlignAllLeft metrics:0
                          views:NSDictionaryOfVariableBindings(imageView, label)]];

    [cell setBackgroundColor: [UIColor greenColor]];
    return cell;
}