GraphicsMagick Invalid bit depth for RGBA image

I want to reduce the colors in arbitrary images to 10. On some images I get a GraphicsMagick exception "Invalid bit depth for RGBA image".

I have noticed a few things while testing on different images.

• On some images GM allows me to set the colors to anythin开发者_开发知识库g over 16, coincidence?
• Some images, it seems that no colors value will work.
• If I disable the opacity channel using +matte, I don't get this error.

I can't see anything in these images to suggest why I am having this problem. I'm hoping someone may have an explanation for this? Any speculation is welcome. Maybe people with experience with ImageMagick might have seen something similar at some point?