macos - How do I draw NSGradient to NSImage?

Question

I'm trying to take an NSGradient and save it as an image in RubyMotion, but I can't get it to work. This is the code I have so far:

gradient = NSGradient.alloc.initWithColors(colors, 
  atLocations: locations.to_pointer(:double), 
  colorSpace: NSColorSpace.genericRGBColorSpace
)

size = Size(width, height)
image = NSImage.imageWithSize(size, flipped: false, drawingHandler: lambda do |rect|
  gradient.drawInRect(rect, angle: angle)
  true
end)

data = image.TIFFRepresentation
data.writeToFile('output.tif', atomically: false)

It runs without error, but the file that is saved is blank and there is no image data. Can anyone help point me in the right direction?