Animatable custom property not working in Swift

I have what I believe is equivalent code--for enabling implicit animation on a custom CALayer subclass property--behaving correctly in Objective-C and incorrectly in Swift.

@implementation CustomLayer
@dynamic thickness;
+(BOOL)needsDisplayForKey:(NSString*)key {
  return [key isEqual: @"thickness"];
}
-(id<CAAction>)actionForKey:(NSString*)key {
  NSLog(@"%@", key);
  if([key isEqual:@"thickness"]) {
  CABasicAnimation *anim = [[CABasicAnimation alloc] init];
  anim.fromValue = [[self presentationLayer] valueForKey:key];
  anim.duration = 2.0;
  return anim;
  } else {
  return [super actionForKey:key];
  }
}
-(void)drawInContext:(nonnull CGContextRef)ctx {
  CGContextSetLineWidth(ctx, [self thickness]);
  CGContextStrokeEllipseInRect(ctx, [self bounds]);
}
@end

// ...


[[self layer] setThickness:20.0]; // this triggers animation as expected

class CustomLayer : CALayer {
  dynamic var thickness : CGFloat = 1.0
  override func drawInContext(ctx: CGContext) {
  CGContextSetLineWidth(ctx, thickness)
  CGContextStrokeEllipseInRect(ctx, bounds)
  }
  override func actionForKey(event: String) -> CAAction? {
  print(event)
  if event == "thickness" {
  let anim = CABasicAnimation()
  anim.fromValue = presentationLayer()?.thickness
  anim.duration = 2.0
  return anim
  } else {
  return super.actionForKey(event)
  }
  }
  class override func needsDisplayForKey(key:String) -> Bool {
  if key == "thickness" {
  return true
  } else {
  return super.needsDisplayForKey(key)
  }
  }
}

// ...

layer.thickness = 20.0 // nothing happens

Am I doing something wrong?