ios - How do I create an alpha only bitmap context -

Apple Tech Note "To create an alpha-only bitmap context simply pass the null to the color scheme parameter."

However, I'm not sure what to pass for the bitmapInfo parameter

I was trying to do something like this, with kCGImageAlpha Only:

  CGContextRef ctx = CGBitmapContextCreate (NULL, scaledSize.width, scaledSize.height, 8, scaledSize.width, NULL, kCGImageAlphaOnly);   
 But it gives me a warning about being wrong. What should I do instead? 
 
 
 The docs clearly states that you can  CGImageAlphaInfo  even if the parameter is < Code> CGBitmapInfo  is defined according to the requirement of a value from enum.  
 A solution value must be entered:  
  (CGITMapInfo) KCGImage Alpha only   
 This will make the compiler happy.