Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
I think the issue is your use of label:. I get the same error using label:. However, it works fine for me if I use -annotate or -draw. IM 6.8.9.7 Q16 Mac OSX
label: and caption: are typically for creating a new image (using -size) of text and not for adding text to an existing image.