ios - CATextLayer : Text is not showed

Question

When I tried to add image as CATextLayer content, text is not shown. Only image is shown. This is my code which I have placed in viewDidLoad() method,

    UIImage *uiimage = [UIImage imageNamed:@"flying.png"];
    CATextLayer *textLayer = [CATextLayer layer];
    textLayer.frame = CGRectMake(144, 42, 76, 21);
    textLayer.font = CFBridgingRetain([UIFont boldSystemFontOfSize:18].fontName);
    textLayer.fontSize = 18;
    textLayer.foregroundColor = [UIColor redColor].CGColor;
    textLayer.backgroundColor = [UIColor yellowColor].CGColor;
    textLayer.alignmentMode = kCAAlignmentCenter;
    textLayer.string = @"Hello world";
    textLayer.contents = (id)uiimage.CGImage;
    [self.view.layer addSublayer:textLayer];

Please help to solve the issue.