CAShapeLayer croping image

I am trying to crop an image using CAShapeLayer using this code

    CALayer* contentLayer = [CALayer layer];
    CAShapeLayer* mask = [CAShapeLayer layer];

    CGMutablePathRef path = CGPathCreateMutable();

    CGPathMoveToPoint(path, NULL, 10, 10);
    CGPathAddLineToPoint(path, NULL, 10, 80);
    CGPathAddLineToPoint(path, NULL, 80, 80);
    CGPathAddLineToPoint(path, NULL, 80, 10);
    mask.path = path;

    [contentLayer setContents:(id)[[UIImage imageNamed:@"image.png"] CGImage]];
    [contentLayer setMask:mask];          

    [[self layer]addSublayer:contentLayer];

After doing this, I see only an empty view;