ICO file loading fails if the file contain embedded PNG

I've created a .ico file with ImageMagick from several PNG files, including 256x256 images, which generate new vista-style .ico files with embedded PNG. Dependending on the order of the images in the file, Qt is unable to load even small images.

I've attached two ico files created with the following ImageMagick command lines:

convert 48.png 32.png 16.png 256.png 128.png ok.ico
convert 256.png 128.png 48.png 32.png 16.png ko.ico

With the ok.ico file, I can properly create a QIcon of 16x16 pixels. With ko.ico, I cannot create a QIcon.