Why animating custom CALayer properties causes other properties to be nil during animation?

I have a custom CALayer (say CircleLayer), containing custom properties (radius and tint). The layer renders itself in its drawInContext: method.

- (void)drawInContext:(CGContextRef)ctx {
    NSLog(@"Drawing layer, tint is %@, radius is %@", self.tint, self.radius);

    CGPoint centerPoint = CGPointMake(CGRectGetWidth(self.bounds)/2, CGRectGetHeight(self.bounds)/2);

    CGContextMoveToPoint(ctx, centerPoint.x, centerPoint.y);
    CGContextAddArc(ctx, centerPoint.x, centerPoint.y, [self.radius doubleValue], radians(0), radians(360), 0);
    CGContextClosePath(ctx);

    /* Filling it */
    CGContextSetFillColorWithColor(ctx, self.tint.CGColor);
    CGContextFillPath(ctx); 
}

I want the radius to be animatable so I've implemented

+ (BOOL)needsDisplayForKey:(NSString *)key {
    if ([key isEqualToString:@"radius"]) {
        return YES;
    }
    return [super needsDisplayForKey:key];
}

And the animation is performed like this:

CABasicAnimation *theAnimation=[CABasicAnimation animationWithKeyPath:@"radius"];
theAnimation.duration=2.0;
theAnimation.fromValue=[NSNumber numberWithDouble:100.0];
theAnimation.toValue=[NSNumber numberWithDouble:50.0];
theAnimation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseInEaseOut];

[circleLayer addAnimation:theAnimation forKey:@"animateRadius"];

circleLayer.radius = [NSNumber numberWithDouble:50.0];

drawInContext: gets called as expected during the animation to redraw the circle, however the tint is set to nil as soon as the animation starts and gets back to its original value when the animation ends.

I've concluded that if I want to animate a custom property and want other properties to keep their value during the animation, I have to animate them too, which I find not being convenient at all.

The purpose is not to grow/shrink a circle, I know I can use transformation for th