Why it's not possible to save a PNG image from Preview when a mask is added? While adding some annotations to a PNG image, using the Preview App, I added a mask to highlight part of the image -- Shapes » Highlight. After doing that I couldn't save the image anymore. I got the following error message:


  The document "[FILE_NAME]" could not be saved.




No detail about what is wrong was given. I tried different file names and directories. Also tried to use "Export" instead of "Save". Nothing worked.

Looking at Apple's support page I couldn't find anything about a specific format or something special about that tool.

I'm wondering what I'm doing wrong.
This is a bug with Preview.

Preview -[CIImage initWithCGImage:options:] failed because the CGImage is nil.
Preview Error: CGImageForSavingWithFlattenedEXIF:depth: failed
Preview finalize:2403: image destination must have at least one image


You can report bugs to Apple at https://bugreport.apple.com.