Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
The syntax is fine. "-set {key} {value}" will accept anything as {key}. It would take more intelligence than IM has to realise that your {key} is very close to, but not exactly, one of the standard attributes.