Animating a CALayer shadowpath

I'm animating the shadow path for CALayer.

The frame resizes correctly, but the shadow does not scale.

Instead the shadow starts at the final size CGSize(20,20) and holds throughout the animation even though I set the shadowPath to an initial value

[CATransaction begin];
[CATransaction setAnimationDuration: 0];
[CATransaction setDisableActions: TRUE];
    layer.frame = CGRectMake(0,0,10,10);
    layer.shadowPath = [UIBezierPath bezierPathWithRect:layer.bounds].CGPath;
[CATransaction commit];

[CATransaction begin];

    [CATransaction setValue:[NSNumber numberWithFloat:10] forKey:kCATransactionAnimationDuration];
    layer.frame = CGRectMake(0,0,20,20);
    layer.shadowPath = [UIBezierPath bezierPathWithRect:tile.bounds].CGPath;

[CATransaction commit];