gist-add-buffer: Invalid Slot Type

Whenever I try to add a new buffer to an existing gist, I get the error:

    byte-code: Invalid slot type: gh-gist-gist, id, string, nil

The weird thing is that this error only seems to occur when I add a buffer that exists on disk.  If I add a buffer like *scratch* or *Messages*, it works just fine.

I created the gist through the dired integration, so some of this is working just fine for me.

Please let me know what sort of debugging information I can provide to assist you.