To be honest, I think you're playing a losing game trying to describe the exact include syntax in an error message. Why not simply say that the include option is invalid, and point people at the syntax: https://developer.mozilla.org/en-US/Add-ons/SDK/High-Level_APIs/page-mod#PageMod%28options%29. Probably someone trying to debug this error will end up here anyway, so why not send them here directly?