Can't swap Puppet art; 'Edit Original' greyed out

I'm usually pretty adept at CA, but this issue has me stumped!

I'm modifying several puppets that I've purchased, and in most cases I've been able to 'Edit Original' and change the artwork in Adobe Illustrator. One of them is giving me problems, however. When I open it up in the RIG window the "Puppet" dropdown in the Properties  panel is blank; normally there's a link to the artwork file. Once more, when I right-click on the Puppet the "Edit Original" command is greyed out. The original AI file is definitely being copied to the Gathered Media folder (and I can modify a copy of it) but I can't update the Puppet art.

Has this happened to anyone else?

TIA,

JVK