Drawing a rectangle in UIGraphics context does not work

I am trying to overlay an image with some extra information: some text and rectangles. Right now I am stuck at drawing the rectangles. They just won't show up. What am I doing wrong? The image itself is being drawn, so the graphics context must be ok.

- (void)drawTag:(NSString *)tag withRect:(CGRect)rect
{
    // Set the color in the current graphics context for future draw operations
    [[UIColor yellowColor] setStroke];
    [[UIColor yellowColor] setFill];

    // Create our drawing path
    UIBezierPath* drawingPath = [UIBezierPath bezierPathWithRect:rect];

    // actually draw it
    [drawingPath stroke];
}

- (IBAction)showDetails:(id)sender
{
    // draw the image
    UIGraphicsBeginImageContext(self.userImage.size);
    // This one shows up:
    [self.userImage开发者_如何学JAVA drawAtPoint:CGPointZero];
    // This one does not:
    [self drawTag:@"Test" withRect:CGRectMake(10, 10, 50, 50)];
    // Show the whole thing:
    self.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    self.imageViewController.modalTransitionStyle = UIModalTransitionStyleFlipHorizontal;
    [self presentModalViewController:self.imageViewController animated:YES];
}

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You should set the lineWidth property of UIBezierPath to a value >= 1